diff --git a/packages/mermaid/src/utils.ts b/packages/mermaid/src/utils.ts
index 767fdaa7d..70e47531f 100644
--- a/packages/mermaid/src/utils.ts
+++ b/packages/mermaid/src/utils.ts
@@ -194,7 +194,10 @@ export const isSubstringInArray = function (str: string, arr: string[]): number
  * @param defaultCurve - The default curve to return
  * @returns The curve factory to use
  */
-export function interpolateToCurve(interpolate?: string, defaultCurve: CurveFactory): CurveFactory {
+export function interpolateToCurve(
+  interpolate: string | undefined,
+  defaultCurve: CurveFactory
+): CurveFactory {
   if (!interpolate) {
     return defaultCurve;
   }