diff --git a/src/Compiler.php b/src/Compiler.php
index f02a4b7e..ac7b56fe 100644
--- a/src/Compiler.php
+++ b/src/Compiler.php
@@ -382,6 +382,11 @@ protected function matchExtendsSingle($rawSingle, &$outOrigin)
         $single = array();
 
         foreach ($rawSingle as $part) {
+            // matches Number
+            if (! is_string($part)) {
+                return false;
+            }
+
             if (! preg_match('/^[\[.:#%]/', $part) && count($single)) {
                 $single[count($single) - 1] .= $part;
             } else {
diff --git a/tests/inputs/directives.scss b/tests/inputs/directives.scss
index 10e39422..14b89cb7 100644
--- a/tests/inputs/directives.scss
+++ b/tests/inputs/directives.scss
@@ -108,3 +108,20 @@ div {
 {
     body { color: purple; background: yellow; }
 }
+
+@keyframes anim-rotate {
+     0% {
+         transform: rotate(0);
+     }
+     100% {
+         transform: rotate(360deg);
+     }
+}
+
+.bold {
+    font-weight: 700;
+}
+
+.icon-ajax {
+    @extend .bold;
+}
diff --git a/tests/outputs/directives.css b/tests/outputs/directives.css
index 2b715582..4a1ced6e 100644
--- a/tests/outputs/directives.css
+++ b/tests/outputs/directives.css
@@ -75,3 +75,13 @@ div {
   body {
     color: purple;
     background: yellow; } }
+
+@keyframes anim-rotate {
+  0% {
+    transform: rotate(0); }
+
+  100% {
+    transform: rotate(360deg); } }
+
+.bold, .icon-ajax {
+  font-weight: 700; }
diff --git a/tests/outputs_numbered/directives.css b/tests/outputs_numbered/directives.css
index 7de43a31..9b77350e 100644
--- a/tests/outputs_numbered/directives.css
+++ b/tests/outputs_numbered/directives.css
@@ -83,3 +83,15 @@ to {
 body {
   color: purple;
   background: yellow; } }
+
+@keyframes anim-rotate {
+/* line 113, inputs/directives.scss */
+0% {
+  transform: rotate(0); }
+/* line 116, inputs/directives.scss */
+100% {
+  transform: rotate(360deg); } }
+/* line 121, inputs/directives.scss */
+.bold, .icon-ajax {
+  font-weight: 700; }
+/* line 125, inputs/directives.scss */