diff --git a/lib/variables.js b/lib/variables.js
index 554c504..15c4ccc 100644
--- a/lib/variables.js
+++ b/lib/variables.js
@@ -15,6 +15,11 @@ const uniqueString = (string) => {
  */
 const replaceVariables = (el,value) => {
 
+  // Guard against undefined or non-string values
+  if (typeof value !== 'string') {
+    return value || '';
+  }
+
   // find non-nested css function calls
   // eg: rgb(...), drop-shadow(...)
 	let funcReg = /([a-z\-]+)\s*\(\s*([^\(\)]*?)\s*(?:,\s*([^\(\)]*?)\s*)?\s*\)/i;