diff --git a/gtk/gtkallocatedbitmask.c b/gtk/gtkallocatedbitmask.c
index dff1189edf..2ebe0e14e2 100644
--- a/gtk/gtkallocatedbitmask.c
+++ b/gtk/gtkallocatedbitmask.c
@@ -207,7 +207,7 @@ _gtk_allocated_bitmask_subtract (GtkBitmask       *mask,
                                  const GtkBitmask *other)
 {
   GtkBitmask other_allocated;
-  guint i;
+  guint i, len;
 
   g_return_val_if_fail (mask != NULL, NULL);
   g_return_val_if_fail (other != NULL, NULL);
@@ -215,9 +215,10 @@ _gtk_allocated_bitmask_subtract (GtkBitmask       *mask,
   mask = gtk_bitmask_ensure_allocated (mask);
   ENSURE_ALLOCATED (other, other_allocated);
 
-  for (i = 0; i < other->len; i++)
+  len = MIN (mask->len, other->len);
+  for (i = 0; i < len; i++)
     {
-      mask->data[i] |= ~other->data[i];
+      mask->data[i] &= ~other->data[i];
     }
 
   return gtk_allocated_bitmask_shrink (mask);
diff --git a/testsuite/gtk/bitmask.c b/testsuite/gtk/bitmask.c
index 7d186a986a..b457c08fd8 100644
--- a/testsuite/gtk/bitmask.c
+++ b/testsuite/gtk/bitmask.c
@@ -280,6 +280,53 @@ test_intersect_hardcoded (void)
     }
 }
 
+static void
+test_subtract_hardcoded (void)
+{
+  GtkBitmask *left, *right, *subtracted, *expected;
+  const char *left_str, *right_str;
+  guint left_len, right_len;
+  guint i, l, r;
+
+  for (l = 0; l < G_N_ELEMENTS (tests); l++)
+    {
+      for (r = 0; r < G_N_ELEMENTS (tests); r++)
+        {
+          left = masks[l];
+          right = masks[r];
+          left_str = tests[l];
+          right_str = tests[r];
+          left_len = strlen (tests[l]);
+          right_len = strlen (tests[r]);
+
+          expected = _gtk_bitmask_new ();
+          for (i = MIN (right_len, left_len); i < left_len; i++)
+            {
+              expected = _gtk_bitmask_set (expected, i, left_str[left_len - i - 1] == '1');
+            }
+          if (left_len > right_len)
+            left_str += left_len - right_len;
+          if (right_len > left_len)
+            right_str += right_len - left_len;
+          i = MIN (right_len, left_len);
+          while (i--)
+            {
+              expected = _gtk_bitmask_set (expected, i, left_str[0] == '1' && right_str[0] == '0');
+              right_str++;
+              left_str++;
+            }
+
+          g_print ("%s - %s\n", _gtk_bitmask_to_string (left), _gtk_bitmask_to_string (right));
+          subtracted = _gtk_bitmask_subtract (_gtk_bitmask_copy (left), right);
+
+          assert_cmpmasks (subtracted, expected);
+
+          _gtk_bitmask_free (subtracted);
+          _gtk_bitmask_free (expected);
+        }
+    }
+}
+
 #define SWAP(_a, _b) G_STMT_START{ \
   guint _tmp = _a; \
   _a = _b; \
@@ -375,6 +422,7 @@ main (int argc, char *argv[])
   g_test_add_func ("/bitmask/union", test_union);
   g_test_add_func ("/bitmask/intersect", test_intersect);
   g_test_add_func ("/bitmask/intersect_hardcoded", test_intersect_hardcoded);
+  g_test_add_func ("/bitmask/subtract_hardcoded", test_subtract_hardcoded);
   g_test_add_func ("/bitmask/invert_range", test_invert_range);
 
   result = g_test_run ();