diff --git a/algovivo/mmgrten/Engine.js b/algovivo/mmgrten/Engine.js
index d4806da4..2170bba7 100644
--- a/algovivo/mmgrten/Engine.js
+++ b/algovivo/mmgrten/Engine.js
@@ -16,7 +16,7 @@ class Engine {
     }
     this.wasmInstance = args.wasmInstance;
     const arr = args.wasmInstance.exports.memory.buffer;
-    const mgr = new mmgr.MemoryManager(arr, args.wasmInstance.exports.__heap_base);
+    const mgr = new mmgr.MemoryManager(arr, Number(args.wasmInstance.exports.__heap_base));
     this.mgr = mgr;
 
     this.functional = this.F = new Functional({
diff --git a/algovivo/mmgrten/mmgr/MemoryManager.js b/algovivo/mmgrten/mmgr/MemoryManager.js
index 618b38dc..887a5496 100644
--- a/algovivo/mmgrten/mmgr/MemoryManager.js
+++ b/algovivo/mmgrten/mmgr/MemoryManager.js
@@ -6,6 +6,7 @@ class MemoryManager {
     this.array = array;
 
     if (heapBase == null) heapBase = 0;
+    else heapBase = Number(heapBase);
 
     this.ptrToSlot = new Map();
 
@@ -121,6 +122,10 @@ class MemoryManager {
 
   free(ptr) {
     const slot = this.ptrToSlot.get(ptr);
+    if (slot == null) {
+      throw new Error(`no slot found for ptr ${ptr}`);
+    }
+    this.ptrToSlot.delete(ptr);
     slot.free();
   }
 }
diff --git a/test/mmgr/memoryManager.test.js b/test/mmgr/memoryManager.test.js
new file mode 100644
index 00000000..200f0e3d
--- /dev/null
+++ b/test/mmgr/memoryManager.test.js
@@ -0,0 +1,12 @@
+const algovivo = require("algovivo");
+
+test("malloc free with non-numeric heapBase", () => {
+  // simulate WebAssembly.Global which has valueOf() but is an object
+  const heapBase = { valueOf: () => 16 };
+  const buffer = new ArrayBuffer(1024);
+  const mgr = new algovivo.mmgrten.mmgr.MemoryManager(buffer, heapBase);
+
+  const ptr = mgr.malloc(32);
+  const numericPtr = Number(ptr);
+  mgr.free(numericPtr);
+});