diff --git a/docs/config/setup/modules/mermaidAPI.md b/docs/config/setup/modules/mermaidAPI.md
index 0406baa81..5049c4885 100644
--- a/docs/config/setup/modules/mermaidAPI.md
+++ b/docs/config/setup/modules/mermaidAPI.md
@@ -95,7 +95,7 @@ mermaid.initialize(config);
 
 #### Defined in
 
-[mermaidAPI.ts:680](https://github.com/mermaid-js/mermaid/blob/master/packages/mermaid/src/mermaidAPI.ts#L680)
+[mermaidAPI.ts:683](https://github.com/mermaid-js/mermaid/blob/master/packages/mermaid/src/mermaidAPI.ts#L683)
 
 ## Functions
 
diff --git a/packages/mermaid/src/mermaidAPI.ts b/packages/mermaid/src/mermaidAPI.ts
index 64e0bbce2..34f26c418 100644
--- a/packages/mermaid/src/mermaidAPI.ts
+++ b/packages/mermaid/src/mermaidAPI.ts
@@ -579,7 +579,10 @@ const render = async function (
 function initialize(options: MermaidConfig = {}) {
   // Handle legacy location of font-family configuration
   if (options?.fontFamily && !options.themeVariables?.fontFamily) {
-    options.themeVariables = { fontFamily: options.fontFamily };
+    if (!options.themeVariables) {
+      options.themeVariables = {};
+    }
+    options.themeVariables.fontFamily = options.fontFamily;
   }
 
   // Set default options