diff --git a/lib/Encoding/UTF16.php b/lib/Encoding/UTF16.php
index 6a60d20..5e4fb32 100644
--- a/lib/Encoding/UTF16.php
+++ b/lib/Encoding/UTF16.php
@@ -87,7 +87,8 @@ abstract class UTF16 extends AbstractEncoding {
     public function asciiSpan(string $mask, int $length = null): string {
         // UTF-16 has no ASCII characters, so we must do things the hard way
         $out = "";
-        while (true) {
+        $left = ($length === null) ? -1 : $length;
+        while ($left) {
             $c1 = @$this->string[$this->posByte];
             $c2 = @$this->string[$this->posByte + 1];
             $b = ord(static::BE ? $c1 : $c2);
@@ -98,6 +99,7 @@ abstract class UTF16 extends AbstractEncoding {
                     $out .= $c;
                     $this->posByte += 2;
                     $this->posChar++;
+                    $left--;
                 } else {
                     break;
                 }
@@ -111,7 +113,8 @@ abstract class UTF16 extends AbstractEncoding {
     public function asciiSpanNot(string $mask, int $length = null): string {
         // this is a copy of asciiSpan above with only the strpos check reversed
         $out = "";
-        while (true) {
+        $left = ($length === null) ? -1 : $length;
+        while ($left) {
             $c1 = @$this->string[$this->posByte];
             $c2 = @$this->string[$this->posByte + 1];
             $b = ord(static::BE ? $c1 : $c2);
@@ -122,6 +125,7 @@ abstract class UTF16 extends AbstractEncoding {
                     $out .= $c;
                     $this->posByte += 2;
                     $this->posChar++;
+                    $left--;
                 } else {
                     break;
                 }
diff --git a/tests/cases/Encoding/TestBig5.php b/tests/cases/Encoding/TestBig5.php
index cab1492..9856c42 100644
--- a/tests/cases/Encoding/TestBig5.php
+++ b/tests/cases/Encoding/TestBig5.php
@@ -204,6 +204,13 @@ class TestBig5 extends \MensBeam\Intl\Test\CoderDecoderTest {
         parent::testExtractAsciiSpans();
     }
 
+    /**
+     * @covers MensBeam\Intl\Encoding\Big5::asciiSpanNot
+     */
+    public function testExtractNegativeAsciiSpans() {
+        parent::testExtractNegativeAsciiSpans();
+    }
+
     /**
      * @group optional
      */
diff --git a/tests/cases/Encoding/TestEUCJP.php b/tests/cases/Encoding/TestEUCJP.php
index ea340d2..178084f 100644
--- a/tests/cases/Encoding/TestEUCJP.php
+++ b/tests/cases/Encoding/TestEUCJP.php
@@ -215,6 +215,13 @@ class TestEUCJP extends \MensBeam\Intl\Test\CoderDecoderTest {
         parent::testExtractAsciiSpans();
     }
 
+    /**
+     * @covers MensBeam\Intl\Encoding\EUCJP::asciiSpanNot
+     */
+    public function testExtractNegativeAsciiSpans() {
+        parent::testExtractNegativeAsciiSpans();
+    }
+
     /**
      * @group optional
      */
diff --git a/tests/cases/Encoding/TestEUCKR.php b/tests/cases/Encoding/TestEUCKR.php
index 2e05f38..3bc0629 100644
--- a/tests/cases/Encoding/TestEUCKR.php
+++ b/tests/cases/Encoding/TestEUCKR.php
@@ -194,6 +194,13 @@ class TestEUCKR extends \MensBeam\Intl\Test\CoderDecoderTest {
         parent::testExtractAsciiSpans();
     }
 
+    /**
+     * @covers MensBeam\Intl\Encoding\EUCKR::asciiSpanNot
+     */
+    public function testExtractNegativeAsciiSpans() {
+        parent::testExtractNegativeAsciiSpans();
+    }
+
     /**
      * @group optional
      */
diff --git a/tests/cases/Encoding/TestGB18030.php b/tests/cases/Encoding/TestGB18030.php
index 89c6bd8..0999e62 100644
--- a/tests/cases/Encoding/TestGB18030.php
+++ b/tests/cases/Encoding/TestGB18030.php
@@ -283,6 +283,13 @@ class TestGB18030 extends \MensBeam\Intl\Test\CoderDecoderTest {
         parent::testExtractAsciiSpans();
     }
 
+    /**
+     * @covers MensBeam\Intl\Encoding\GB18030::asciiSpanNot
+     */
+    public function testExtractNegativeAsciiSpans() {
+        parent::testExtractNegativeAsciiSpans();
+    }
+
     /**
      * @group optional
      */
diff --git a/tests/cases/Encoding/TestISO2022JP.php b/tests/cases/Encoding/TestISO2022JP.php
index 399825e..8be3d07 100644
--- a/tests/cases/Encoding/TestISO2022JP.php
+++ b/tests/cases/Encoding/TestISO2022JP.php
@@ -250,6 +250,13 @@ class TestISO2022JP extends \MensBeam\Intl\Test\CoderDecoderTest {
         $this->markTestIncomplete();
     }
 
+    /**
+     * @covers MensBeam\Intl\Encoding\ISO2022JP::asciiSpanNot
+     */
+    public function testExtractNegativeAsciiSpans() {
+        $this->markTestIncomplete();
+    }
+
     /**
      * @group optional
      */
diff --git a/tests/cases/Encoding/TestReplacement.php b/tests/cases/Encoding/TestReplacement.php
index d4fe70c..dae687a 100644
--- a/tests/cases/Encoding/TestReplacement.php
+++ b/tests/cases/Encoding/TestReplacement.php
@@ -203,6 +203,19 @@ class TestReplacement extends \MensBeam\Intl\Test\DecoderTest {
      * @covers MensBeam\Intl\Encoding\Replacement::asciiSpan
      */
     public function testExtractAsciiSpans() {
-        $this->markTestIncomplete();
+        $d = new Replacement("VVVVVV");
+        $this->assertSame("", $d->asciiSpan($this->allBytes()));
+        $d->nextChar();
+        $this->assertTrue($d->eof());
+    }
+
+    /**
+     * @covers MensBeam\Intl\Encoding\Replacement::asciiSpanNot
+     */
+    public function testExtractNegativeAsciiSpans() {
+        $d = new Replacement("VVVVVV");
+        $this->assertSame("", $d->asciiSpanNot(""));
+        $d->nextChar();
+        $this->assertTrue($d->eof());
     }
 }
diff --git a/tests/cases/Encoding/TestShiftJIS.php b/tests/cases/Encoding/TestShiftJIS.php
index 0b56f07..8945426 100644
--- a/tests/cases/Encoding/TestShiftJIS.php
+++ b/tests/cases/Encoding/TestShiftJIS.php
@@ -207,6 +207,13 @@ class TestShiftJIS extends \MensBeam\Intl\Test\CoderDecoderTest {
         parent::testExtractAsciiSpans();
     }
 
+    /**
+     * @covers MensBeam\Intl\Encoding\ShiftJIS::asciiSpanNot
+     */
+    public function testExtractNegativeAsciiSpans() {
+        parent::testExtractNegativeAsciiSpans();
+    }
+
     /**
      * @group optional
      */
diff --git a/tests/cases/Encoding/TestSingleByte.php b/tests/cases/Encoding/TestSingleByte.php
index a3917d0..a957feb 100644
--- a/tests/cases/Encoding/TestSingleByte.php
+++ b/tests/cases/Encoding/TestSingleByte.php
@@ -237,12 +237,23 @@ class TestSingleByte extends \MensBeam\Intl\Test\CoderDecoderTest {
     }
 
     /**
+     * @dataProvider provideClasses
      * @covers MensBeam\Intl\Encoding\SingleByteEncoding::asciiSpan
      */
-    public function testExtractAsciiSpans() {
+    public function testExtractAsciiSpans($class =  null) {
+        $this->testedClass = $class;
         parent::testExtractAsciiSpans();
     }
 
+    /**
+     * @dataProvider provideClasses
+     * @covers MensBeam\Intl\Encoding\SingleByteEncoding::asciiSpan
+     */
+    public function testExtractNegativeAsciiSpans($class =  null) {
+        $this->testedClass = $class;
+        parent::testExtractNegativeAsciiSpans();
+    }
+
     public function provideClasses() {
         foreach (self::$classes as $name => $class) {
             yield $name => [$class];
diff --git a/tests/cases/Encoding/TestUTF16LE.php b/tests/cases/Encoding/TestUTF16LE.php
index 6cf4bbf..ea9d50d 100644
--- a/tests/cases/Encoding/TestUTF16LE.php
+++ b/tests/cases/Encoding/TestUTF16LE.php
@@ -142,6 +142,13 @@ class TestUTF16LE extends \MensBeam\Intl\Test\DecoderTest {
         parent::testExtractAsciiSpans();
     }
 
+    /**
+     * @covers MensBeam\Intl\Encoding\UTF16::asciiSpanNot
+     */
+    public function testExtractNegativeAsciiSpans() {
+        parent::testExtractNegativeAsciiSpans();
+    }
+
     public function provideStrings() {
         return [
             // control samples
diff --git a/tests/cases/Encoding/TestUTF8.php b/tests/cases/Encoding/TestUTF8.php
index 1af1839..80685eb 100644
--- a/tests/cases/Encoding/TestUTF8.php
+++ b/tests/cases/Encoding/TestUTF8.php
@@ -233,4 +233,11 @@ class TestUTF8 extends \MensBeam\Intl\Test\CoderDecoderTest {
     public function testExtractAsciiSpans() {
         parent::testExtractAsciiSpans();
     }
+
+    /**
+     * @covers MensBeam\Intl\Encoding\UTF8::asciiSpanNot
+     */
+    public function testExtractNegativeAsciiSpans() {
+        parent::testExtractNegativeAsciiSpans();
+    }
 }
diff --git a/tests/cases/Encoding/TestXUserDefined.php b/tests/cases/Encoding/TestXUserDefined.php
index 7f196fd..658b130 100644
--- a/tests/cases/Encoding/TestXUserDefined.php
+++ b/tests/cases/Encoding/TestXUserDefined.php
@@ -18,6 +18,8 @@ class TestXUserDefined extends \MensBeam\Intl\Test\CoderDecoderTest {
     protected $seekOffsets = [0, 1, 2, 3, 4, 5, 6, 7];
     /* This string is supposed to contain an invalid character sequence sandwiched between two null characters, but x-user-defined has no invalid characters */
     protected $brokenChar = "";
+    /* This string conatins the ASCII characters "A" and "Z" followed by two arbitrary non-ASCII characters, followed by the two ASCII characters "0" and "9" */
+    protected $spanString = "41 5A 80 FF 30 39";
 
     public function provideCodePoints() {
         return [
@@ -190,4 +192,11 @@ class TestXUserDefined extends \MensBeam\Intl\Test\CoderDecoderTest {
     public function testExtractAsciiSpans() {
         parent::testExtractAsciiSpans();
     }
+
+    /**
+     * @covers MensBeam\Intl\Encoding\XUserDefined::asciiSpanNot
+     */
+    public function testExtractNegativeAsciiSpans() {
+        parent::testExtractNegativeAsciiSpans();
+    }
 }
diff --git a/tests/lib/DecoderTest.php b/tests/lib/DecoderTest.php
index 451edc8..8591b9b 100644
--- a/tests/lib/DecoderTest.php
+++ b/tests/lib/DecoderTest.php
@@ -368,6 +368,19 @@ abstract class DecoderTest extends \PHPUnit\Framework\TestCase {
         $this->assertSame("09", $d->asciiSpan($allBytes));
     }
 
+    public function testExtractNegativeAsciiSpans() {
+        $class = $this->testedClass;
+        $d = new $class($this->prepString($this->spanString));
+        $this->assertSame("", $d->asciiSpanNot("AZ"));
+        $this->assertSame("A", $d->asciiSpanNot("az", 1));
+        $this->assertSame("Z", $d->asciiSpanNot("az"));
+        $this->assertSame("", $d->asciiSpanNot(""));
+        $d->nextChar();
+        $this->assertSame("", $d->asciiSpanNot(""));
+        $d->nextChar();
+        $this->assertSame("09", $d->asciiSpanNot(""));
+    }
+
     protected function prepString(string $str): string {
         return hex2bin(str_replace(" ", "", $str));
     }