diff --git a/packages/mdc-textfield/_mixins.scss b/packages/mdc-textfield/_mixins.scss
index 97067dbad31..d1fb9d47980 100644
--- a/packages/mdc-textfield/_mixins.scss
+++ b/packages/mdc-textfield/_mixins.scss
@@ -60,7 +60,7 @@
     // background between the two borders in each corner when the
     // textarea is focused. This also means we need to guard against
     // invalid output.
-    $radius-value: mdc-shape-prop-value($radius);
+    $radius-value: mdc-shape-prop-value_($radius);
     $trimmed-radius-value: ();
 
     @each $corner in $radius-value {