diff --git a/src/runtime/python/ffi.c b/src/runtime/python/ffi.c
index d308850a0..3d7ec5598 100644
--- a/src/runtime/python/ffi.c
+++ b/src/runtime/python/ffi.c
@@ -27,7 +27,7 @@ PgfExnType handleError(PgfExn err)
 }
 
 // ----------------------------------------------------------------------------
-// string conversions
+// conversions
 
 PgfText *
 PyUnicode_AsPgfText(PyObject *pystr)
@@ -54,6 +54,51 @@ PyUnicode_FromPgfText(PgfText *text)
     return PyUnicode_FromStringAndSize(text->text, text->size);
 }
 
+PgfTypeHypo *
+PyList_AsHypos(PyObject *pylist, Py_ssize_t *n_hypos)
+{
+    if (!PyList_Check(pylist)) {
+        PyErr_SetString(PyExc_TypeError, "PyList_AsHypos: must be a list");
+        return NULL;
+    }
+    Py_ssize_t n = PyList_Size(pylist);
+    *n_hypos = n;
+    PgfTypeHypo *hypos = PyMem_Malloc(sizeof(PgfTypeHypo)*n);
+
+    for (Py_ssize_t i = 0; i < n; i++) {
+        PyObject *hytup = PyList_GetItem(pylist, i);
+        if (!PyTuple_Check(hytup)) {
+            PyErr_SetString(PyExc_TypeError, "PyList_AsHypos: item must be a tuple");
+            return NULL;
+        }
+
+        PyObject *t0 = PyTuple_GetItem(hytup, 0);
+        if (!PyLong_Check(t0)) {
+            PyErr_SetString(PyExc_TypeError, "PyList_AsHypos: first element must be an integer");
+            return NULL;
+        }
+        hypos[i].bind_type = PyLong_AsLong(t0);
+
+        PyObject *t1 = PyTuple_GetItem(hytup, 1);
+        if (!PyUnicode_Check(t1)) {
+            PyErr_SetString(PyExc_TypeError, "PyList_AsHypos: second element must be a string");
+            return NULL;
+        }
+        hypos[i].cid = PyUnicode_AsPgfText(t1);
+
+        PyObject *t2 = PyTuple_GetItem(hytup, 2);
+        if (!PyObject_TypeCheck(t2, &pgf_TypeType)) {
+            PyErr_SetString(PyExc_TypeError, "PyList_AsHypos: third element must be a Type");
+            return NULL;
+        }
+        hypos[i].type = (PgfType) t2;
+        Py_INCREF(hypos[i].type);
+    }
+
+    return hypos;
+}
+
+
 // ----------------------------------------------------------------------------
 // unmarshaller
 
@@ -322,16 +367,10 @@ match_type(PgfMarshaller *this, PgfUnmarshaller *u, PgfType ty)
 {
     TypeObject *type = (TypeObject *)ty;
 
-    Py_ssize_t n_hypos = PyList_Size(type->hypos);
-    PgfTypeHypo hypos[n_hypos];
-    // PgfTypeHypo *hypos = alloca(sizeof(PgfTypeHypo)*n_hypos);
-    for (Py_ssize_t i = 0; i < n_hypos; i++) {
-        PyObject *hytup = PyList_GetItem(type->hypos, i);
-        hypos[i].bind_type = PyLong_AsLong(PyTuple_GetItem(hytup, 0));
-        hypos[i].cid = PyUnicode_AsPgfText(PyTuple_GetItem(hytup, 1));
-        hypos[i].type = (PgfType) PyTuple_GetItem(hytup, 2);
-        Py_INCREF(hypos[i].type);
-    }
+    Py_ssize_t n_hypos;
+    PgfTypeHypo *hypos = PyList_AsHypos(type->hypos, &n_hypos);
+    if (PyErr_Occurred())
+        return 0;
 
     PgfText *cat = PyUnicode_AsPgfText(type->cat);
     if (cat == NULL) {
@@ -340,7 +379,6 @@ match_type(PgfMarshaller *this, PgfUnmarshaller *u, PgfType ty)
 
     Py_ssize_t n_exprs = PyList_Size(type->exprs);
     PgfExpr exprs[n_exprs];
-    // PgfExpr *exprs = alloca(sizeof(PgfExpr)*n_exprs);
     for (Py_ssize_t i = 0; i < n_exprs; i++) {
         exprs[i] = (PgfExpr)PyList_GetItem(type->exprs, i);
         Py_INCREF(exprs[i]);
diff --git a/src/runtime/python/ffi.h b/src/runtime/python/ffi.h
index 7197c509e..613ba9721 100644
--- a/src/runtime/python/ffi.h
+++ b/src/runtime/python/ffi.h
@@ -18,6 +18,8 @@ PgfExnType handleError(PgfExn err);
 PgfText *PyUnicode_AsPgfText(PyObject *pystr);
 PyObject *PyUnicode_FromPgfText(PgfText *text);
 
+PgfTypeHypo *PyList_AsHypos(PyObject *pylist, Py_ssize_t *n_hypos);
+
 PgfUnmarshaller unmarshaller;
 PgfMarshaller marshaller;
 
diff --git a/src/runtime/python/transactions.c b/src/runtime/python/transactions.c
index 30517d33a..64f1bda01 100644
--- a/src/runtime/python/transactions.c
+++ b/src/runtime/python/transactions.c
@@ -116,21 +116,16 @@ Transaction_createCategory(TransactionObject *self, PyObject *args)
         PyErr_SetString(PyExc_TypeError, "hypos must be a list");
         return NULL;
     }
+    // Py_INCREF(hypos);
 
     PgfText *catname = (PgfText *)PyMem_Malloc(sizeof(PgfText)+size+1);
     memcpy(catname->text, s, size+1);
     catname->size = size;
 
-    Py_ssize_t n_hypos = PyList_Size(hypos);
-    PgfTypeHypo context[n_hypos];
-    // PgfTypeHypo *context = alloca(sizeof(PgfTypeHypo)*n_hypos);
-    for (Py_ssize_t i = 0; i < n_hypos; i++) {
-        PyObject *hytup = PyList_GetItem(hypos, i);
-        context[i].bind_type = PyLong_AsLong(PyTuple_GetItem(hytup, 0));
-        context[i].cid = PyUnicode_AsPgfText(PyTuple_GetItem(hytup, 1));
-        context[i].type = (PgfType) PyTuple_GetItem(hytup, 2);
-        Py_INCREF(context[i].type);
-    }
+    Py_ssize_t n_hypos;
+    PgfTypeHypo *context = PyList_AsHypos(hypos, &n_hypos);
+    if (PyErr_Occurred())
+        return NULL;
 
     PgfExn err;
     pgf_create_category(self->pgf->db, self->revision, catname, n_hypos, context, prob, &marshaller, &err);