diff --git a/js/src/ctypes/CTypes.cpp b/js/src/ctypes/CTypes.cpp
index 6a29f769f3fb..48f5177613ec 100644
--- a/js/src/ctypes/CTypes.cpp
+++ b/js/src/ctypes/CTypes.cpp
@@ -6850,15 +6850,12 @@ CreateFunctionInfo(JSContext* cx,
                    HandleObject returnType,
                    const HandleValueArray& args)
 {
-  FunctionInfo* fninfo(cx->new_<FunctionInfo>());
+  auto fninfo = cx->make_unique<FunctionInfo>();
   if (!fninfo) {
     JS_ReportOutOfMemory(cx);
     return false;
   }
 
-  // Stash the FunctionInfo in a reserved slot.
-  JS_SetReservedSlot(typeObj, SLOT_FNINFO, PrivateValue(fninfo));
-
   ffi_abi abi;
   if (!GetABI(cx, abiType, &abi)) {
     JS_ReportErrorASCII(cx, "Invalid ABI specification");
@@ -6868,7 +6865,7 @@ CreateFunctionInfo(JSContext* cx,
 
   fninfo->mReturnType = returnType;
 
-  // prepare the argument types
+  // Prepare the argument types.
   if (!fninfo->mArgTypes.reserve(args.length()) ||
       !fninfo->mFFITypes.reserve(args.length())) {
     JS_ReportOutOfMemory(cx);
@@ -6913,14 +6910,12 @@ CreateFunctionInfo(JSContext* cx,
     fninfo->mFFITypes.infallibleAppend(ffiType);
   }
 
-  if (fninfo->mIsVariadic) {
-    // wait to PrepareCIF until function is called
-    return true;
-  }
-
-  if (!PrepareCIF(cx, fninfo))
+  // For variadic functions, wait to PrepareCIF until the function is called.
+  if (!fninfo->mIsVariadic && !PrepareCIF(cx, fninfo.get()))
     return false;
 
+  // Now the FunctionInfo has been initialized, store it in a reserved slot.
+  JS_SetReservedSlot(typeObj, SLOT_FNINFO, PrivateValue(fninfo.release()));
   return true;
 }