diff --git a/src/styleq.js b/src/styleq.js
index 7089007..e3cfe4f 100644
--- a/src/styleq.js
+++ b/src/styleq.js
@@ -38,7 +38,7 @@ function createStyleq(options?: StyleqOptions): Styleq {
 
   return function styleq() {
     // Keep track of property commits to the className
-    const definedProperties = [];
+    const definedProperties: Array<string> = [];
     // The className and inline style to build up
     let className = '';
     let inlineStyle: null | InlineStyle = null;
@@ -100,14 +100,13 @@ function createStyleq(options?: StyleqOptions): Styleq {
                 const propsToDefine = transformProperty
                   ? transformProperty(prop)
                   : prop;
-                const propsIsArray = Array.isArray(propsToDefine);
-                if (propsIsArray) {
+                if (Array.isArray(propsToDefine)) {
                   definedProperties.push(...propsToDefine);
                 } else {
                   definedProperties.push(propsToDefine);
                 }
                 if (nextCache != null) {
-                  if (propsIsArray) {
+                  if (Array.isArray(propsToDefine)) {
                     definedPropertiesChunk.push(...propsToDefine);
                   } else {
                     definedPropertiesChunk.push(propsToDefine);
diff --git a/styleq.flow.js b/styleq.flow.js
index c697fb2..a648596 100644
--- a/styleq.flow.js
+++ b/styleq.flow.js
@@ -27,7 +27,7 @@ export type StyleqOptions = {
   disableCache?: boolean,
   disableMix?: boolean,
   transform?: (EitherStyle) => EitherStyle,
-  transformProperty?: (string) => $ReadOnlyArray<string>,
+  transformProperty?: (string) => $ReadOnlyArray<string> | string,
 };
 
 export type StyleqResult = [string, InlineStyle | null];