diff --git a/include/cyclone/runtime.h b/include/cyclone/runtime.h
index 7a8cbc2b..ce05dc43 100644
--- a/include/cyclone/runtime.h
+++ b/include/cyclone/runtime.h
@@ -216,7 +216,7 @@ port_type Cyc_stdin(void);
 port_type Cyc_stderr(void);
 port_type Cyc_io_open_input_file(void *data, object str);
 port_type Cyc_io_open_output_file(void *data, object str);
-port_type Cyc_io_open_output_string(void *data);
+port_type *Cyc_io_open_output_string(void *data);
 void Cyc_io_get_output_string(void *data, object cont, object port);
 object Cyc_io_close_port(void *data, object port);
 object Cyc_io_close_input_port(void *data, object port);
diff --git a/mstreams.c b/mstreams.c
index 5ea1b5f0..da232716 100644
--- a/mstreams.c
+++ b/mstreams.c
@@ -33,14 +33,24 @@ if (type_is_pair_prim(clo)) { \
  } \
 }
 
-port_type Cyc_io_open_output_string(void *data)
+object Cyc_heap_alloc_port(void *data);
+port_type *Cyc_io_open_output_string(void *data)
 {
-  make_port(p, NULL, 0);
+  // Allocate port on the heap so the location of mem_buf does not change
+  // make_port(p, NULL, 0);
+  port_type *p = (port_type *)Cyc_heap_alloc_port(data);
+  p->hdr.mark = ((gc_thread_data *)data)->gc_alloc_color;
+  p->hdr.grayed = 0;
+  p->tag = port_tag;
+  p->fp = NULL;
+  p->mode = 0; // Output
+  p->mem_buf = NULL;
+  p->mem_buf_len = 0;
   errno = 0;
 #if CYC_HAVE_OPEN_MEMSTREAM
-  p.fp = open_memstream(&(p.mem_buf), &(p.mem_buf_len));
+  p->fp = open_memstream(&(p->mem_buf), &(p->mem_buf_len));
 #endif
-  if (p.fp == NULL){
+  if (p->fp == NULL){
     Cyc_rt_raise2(data, "Unable to open memory stream", obj_int2obj(errno));
   }
   return p;
diff --git a/runtime.c b/runtime.c
index be6f19b8..b2973d0e 100644
--- a/runtime.c
+++ b/runtime.c
@@ -926,6 +926,19 @@ object Cyc_write_char(void *data, object c, object port)
   return quote_void;
 }
 
+// Internal function, do not use this anywhere outside the runtime
+object Cyc_heap_alloc_port(void *data)
+{
+  object p = NULL;
+  int heap_grown;
+  p = gc_alloc(Cyc_heap, 
+               sizeof(port_type),
+               boolean_f, // OK to populate manually over here
+               (gc_thread_data *)data, 
+               &heap_grown);
+  return p;
+}
+
 // TODO: should not be a predicate, may end up moving these to Scheme code
 object memberp(void *data, object x, list l)
 {
diff --git a/scheme/base.sld b/scheme/base.sld
index fca004da..d4feaef3 100644
--- a/scheme/base.sld
+++ b/scheme/base.sld
@@ -1163,8 +1163,8 @@
        ((p->mode == 0 && p->fp != NULL) ? boolean_t : boolean_f)); ")
   (define-c open-output-string
     "(void *data, int argc, closure _, object k)"
-    " port_type p = Cyc_io_open_output_string(data);
-      return_closcall1(data, k, &p); ")
+    " port_type *p = Cyc_io_open_output_string(data);
+      return_closcall1(data, k, p); ")
   (define-c get-output-string
     "(void *data, int argc, closure _, object k, object port)"
     " Cyc_io_get_output_string(data, k, port);