diff --git a/ext/zlib/zlib.c b/ext/zlib/zlib.c
index bcdd32a..fcd4928 100644
--- a/ext/zlib/zlib.c
+++ b/ext/zlib/zlib.c
@@ -532,8 +532,14 @@ rb_zlib_crc32(int argc, VALUE *argv, VALUE klass)
 static VALUE
 rb_zlib_crc32_combine(VALUE klass, VALUE crc1, VALUE crc2, VALUE len2)
 {
+    long len = NUM2LONG(len2);
+
+    /* zlib 1.3.1 loops forever on a negative length */
+    if (len < 0) {
+	rb_raise(rb_eArgError, "negative length");
+    }
     return ULONG2NUM(
-	crc32_combine(NUM2ULONG(crc1), NUM2ULONG(crc2), NUM2LONG(len2)));
+	crc32_combine(NUM2ULONG(crc1), NUM2ULONG(crc2), len));
 }
 #else
 #define rb_zlib_crc32_combine rb_f_notimplement
diff --git a/test/zlib/test_zlib.rb b/test/zlib/test_zlib.rb
index e1ecbf8..0794f9a 100644
--- a/test/zlib/test_zlib.rb
+++ b/test/zlib/test_zlib.rb
@@ -1634,6 +1634,12 @@ def test_crc32_combine
       end
     end
 
+    def test_crc32_combine_negative_length
+      assert_raise(ArgumentError) { Zlib.crc32_combine(0, 0, -1) }
+    rescue NotImplementedError
+      omit "crc32_combine is not implemented"
+    end
+
     def test_crc_table
       t = Zlib.crc_table
       assert_instance_of(Array, t)