diff --git a/src/builtins.c b/src/builtins.c
index a48a0fb84e297..cdc9ed426a11e 100644
--- a/src/builtins.c
+++ b/src/builtins.c
@@ -1534,7 +1534,7 @@ JL_DLLEXPORT size_t jl_static_show(JL_STREAM *out, jl_value_t *v)
 
 JL_DLLEXPORT size_t jl_static_show_func_sig(JL_STREAM *s, jl_value_t *type)
 {
-    jl_value_t *ftype = jl_first_argument_datatype(type);
+    jl_value_t *ftype = (jl_value_t*)jl_first_argument_datatype(type);
     if (ftype == NULL)
         return jl_static_show(s, type);
     size_t n = 0;
diff --git a/src/dump.c b/src/dump.c
index 00813dba3bcb4..c5e14b00bd379 100644
--- a/src/dump.c
+++ b/src/dump.c
@@ -1691,8 +1691,8 @@ static void jl_deserialize_lambdas_from_mod(ios_t *s)
         if (meth == NULL)
             return;
         jl_tupletype_t *simpletype = (jl_tupletype_t*)jl_deserialize_value(s, NULL);
-        jl_datatype_t *gf = (jl_datatype_t*)jl_tparam0(meth->sig);
-        assert(jl_is_datatype(gf));
+        jl_datatype_t *gf = jl_first_argument_datatype((jl_value_t*)meth->sig);
+        assert(jl_is_datatype(gf) && gf->name->mt);
         jl_method_table_insert(gf->name->mt, meth, simpletype);
     }
 }
diff --git a/src/julia_internal.h b/src/julia_internal.h
index 356f1d08589f8..e22529569e70a 100644
--- a/src/julia_internal.h
+++ b/src/julia_internal.h
@@ -212,7 +212,7 @@ jl_lambda_info_t *jl_method_lookup_by_type(jl_methtable_t *mt, jl_tupletype_t *t
 jl_lambda_info_t *jl_method_lookup(jl_methtable_t *mt, jl_value_t **args, size_t nargs, int cache);
 jl_value_t *jl_gf_invoke(jl_tupletype_t *types, jl_value_t **args, size_t nargs);
 
-jl_value_t *jl_first_argument_datatype(jl_value_t *argtypes);
+jl_datatype_t *jl_first_argument_datatype(jl_value_t *argtypes);
 int jl_has_intrinsics(jl_lambda_info_t *li, jl_value_t *v, jl_module_t *m);
 
 jl_value_t *jl_nth_slot_type(jl_tupletype_t *sig, size_t i);
diff --git a/src/toplevel.c b/src/toplevel.c
index b08800f498f70..e6879ed226f37 100644
--- a/src/toplevel.c
+++ b/src/toplevel.c
@@ -661,7 +661,7 @@ JL_DLLEXPORT jl_value_t *jl_generic_function_def(jl_sym_t *name, jl_value_t **bp
     return gf;
 }
 
-jl_datatype_t *first_arg_datatype(jl_value_t *a, int got_tuple1)
+static jl_datatype_t *first_arg_datatype(jl_value_t *a, int got_tuple1)
 {
     if (jl_is_datatype(a)) {
         if (got_tuple1)
@@ -696,9 +696,9 @@ jl_datatype_t *first_arg_datatype(jl_value_t *a, int got_tuple1)
 }
 
 // get DataType of first tuple element, or NULL if cannot be determined
-jl_value_t *jl_first_argument_datatype(jl_value_t *argtypes)
+jl_datatype_t *jl_first_argument_datatype(jl_value_t *argtypes)
 {
-    return (jl_value_t*)first_arg_datatype(argtypes, 0);
+    return first_arg_datatype(argtypes, 0);
 }
 
 extern tracer_cb jl_newmeth_tracer;
@@ -726,17 +726,17 @@ JL_DLLEXPORT void jl_method_def(jl_svec_t *argdata, jl_lambda_info_t *f, jl_valu
 
     if (jl_is_tuple_type(argtypes) && jl_nparams(argtypes) > 0 && !jl_is_type(jl_tparam0(argtypes)))
         jl_error("function type in method definition is not a type");
-    jl_value_t *ftype = jl_first_argument_datatype((jl_value_t*)argtypes);
+    jl_datatype_t *ftype = jl_first_argument_datatype((jl_value_t*)argtypes);
     if (ftype == NULL ||
-        !(jl_is_type_type(ftype) ||
+        !(jl_is_type_type((jl_value_t*)ftype) ||
           (jl_is_datatype(ftype) &&
-           (!((jl_datatype_t*)ftype)->abstract || jl_is_leaf_type(ftype)) &&
-           ((jl_datatype_t*)ftype)->name->mt != NULL)))
+           (!ftype->abstract || jl_is_leaf_type((jl_value_t*)ftype)) &&
+           ftype->name->mt != NULL)))
         jl_error("cannot add methods to an abstract type");
-    mt = ((jl_datatype_t*)ftype)->name->mt;
+    mt = ftype->name->mt;
     name = mt->name;
 
-    if (jl_subtype(ftype, (jl_value_t*)jl_builtin_type, 0))
+    if (jl_subtype((jl_value_t*)ftype, (jl_value_t*)jl_builtin_type, 0))
         jl_error("cannot add methods to a builtin function");
 
     m = jl_new_method(f, name, argtypes, tvars, isstaged == jl_true);