diff --git a/bin/ruby-build b/bin/ruby-build
index 1ec4709025..7652345adc 100755
--- a/bin/ruby-build
+++ b/bin/ruby-build
@@ -368,7 +368,7 @@ fetch_tarball() {
     elif [ -n "$RUBY_BUILD_MIRROR_PACKAGE_URL" ]; then
       mirror_url="$RUBY_BUILD_MIRROR_PACKAGE_URL"
     elif [ -n "$RUBY_BUILD_MIRROR_CMD" ]; then
-      mirror_url="$("$RUBY_BUILD_MIRROR_CMD" "$package_url" "$checksum")"
+      mirror_url="$(command "$RUBY_BUILD_MIRROR_CMD" "$package_url" "$checksum")"
     fi
   fi