diff --git a/internal/cgen/func.go b/internal/cgen/func.go
index 1996e78..3f8daf5 100644
--- a/internal/cgen/func.go
+++ b/internal/cgen/func.go
@@ -420,13 +420,10 @@
 	for _, o := range n.In().Fields() {
 		o := o.AsField()
 		oTyp := o.XType()
-		if oTyp.Decorator() != t.IDPtr && !oTyp.IsRefined() {
-			// TODO: Also check elements, for array-typed arguments.
-			continue
-		}
 
+		// TODO: Also check elements, for array-typed arguments.
 		switch {
-		case oTyp.Decorator() == t.IDPtr:
+		case oTyp.IsIOType() || (oTyp.Decorator() == t.IDPtr):
 			checks = append(checks, fmt.Sprintf("!%s%s", aPrefix, o.Name().Str(g.tm)))
 
 		case oTyp.IsRefined():
diff --git a/release/c/wuffs-unsupported-snapshot.c b/release/c/wuffs-unsupported-snapshot.c
index 99ed30c..ebe1154 100644
--- a/release/c/wuffs-unsupported-snapshot.c
+++ b/release/c/wuffs-unsupported-snapshot.c
@@ -6384,6 +6384,10 @@
                ? wuffs_base__error__disabled_by_previous_error
                : wuffs_base__error__initialize_not_called;
   }
+  if (!a_dst || !a_src) {
+    self->private_impl.magic = WUFFS_BASE__DISABLED;
+    return wuffs_base__error__bad_argument;
+  }
   if ((self->private_impl.active_coroutine != 0) &&
       (self->private_impl.active_coroutine != 1)) {
     self->private_impl.magic = WUFFS_BASE__DISABLED;
@@ -8135,6 +8139,10 @@
                ? wuffs_base__error__disabled_by_previous_error
                : wuffs_base__error__initialize_not_called;
   }
+  if (!a_dst || !a_src) {
+    self->private_impl.magic = WUFFS_BASE__DISABLED;
+    return wuffs_base__error__bad_argument;
+  }
   if ((self->private_impl.active_coroutine != 0) &&
       (self->private_impl.active_coroutine != 1)) {
     self->private_impl.magic = WUFFS_BASE__DISABLED;
@@ -8730,6 +8738,10 @@
                ? wuffs_base__error__disabled_by_previous_error
                : wuffs_base__error__initialize_not_called;
   }
+  if (!a_src) {
+    self->private_impl.magic = WUFFS_BASE__DISABLED;
+    return wuffs_base__error__bad_argument;
+  }
   if ((self->private_impl.active_coroutine != 0) &&
       (self->private_impl.active_coroutine != 1)) {
     self->private_impl.magic = WUFFS_BASE__DISABLED;
@@ -8843,6 +8855,10 @@
                ? wuffs_base__error__disabled_by_previous_error
                : wuffs_base__error__initialize_not_called;
   }
+  if (!a_src) {
+    self->private_impl.magic = WUFFS_BASE__DISABLED;
+    return wuffs_base__error__bad_argument;
+  }
   if ((self->private_impl.active_coroutine != 0) &&
       (self->private_impl.active_coroutine != 2)) {
     self->private_impl.magic = WUFFS_BASE__DISABLED;
@@ -9097,6 +9113,10 @@
                ? wuffs_base__error__disabled_by_previous_error
                : wuffs_base__error__initialize_not_called;
   }
+  if (!a_src) {
+    self->private_impl.magic = WUFFS_BASE__DISABLED;
+    return wuffs_base__error__bad_argument;
+  }
   if ((self->private_impl.active_coroutine != 0) &&
       (self->private_impl.active_coroutine != 3)) {
     self->private_impl.magic = WUFFS_BASE__DISABLED;
@@ -9362,7 +9382,7 @@
                ? wuffs_base__error__disabled_by_previous_error
                : wuffs_base__error__initialize_not_called;
   }
-  if (!a_dst) {
+  if (!a_dst || !a_src) {
     self->private_impl.magic = WUFFS_BASE__DISABLED;
     return wuffs_base__error__bad_argument;
   }
@@ -11263,6 +11283,10 @@
                ? wuffs_base__error__disabled_by_previous_error
                : wuffs_base__error__initialize_not_called;
   }
+  if (!a_dst || !a_src) {
+    self->private_impl.magic = WUFFS_BASE__DISABLED;
+    return wuffs_base__error__bad_argument;
+  }
   if ((self->private_impl.active_coroutine != 0) &&
       (self->private_impl.active_coroutine != 1)) {
     self->private_impl.magic = WUFFS_BASE__DISABLED;
@@ -11740,6 +11764,10 @@
                ? wuffs_base__error__disabled_by_previous_error
                : wuffs_base__error__initialize_not_called;
   }
+  if (!a_dst || !a_src) {
+    self->private_impl.magic = WUFFS_BASE__DISABLED;
+    return wuffs_base__error__bad_argument;
+  }
   if ((self->private_impl.active_coroutine != 0) &&
       (self->private_impl.active_coroutine != 1)) {
     self->private_impl.magic = WUFFS_BASE__DISABLED;