diff --git a/src/mono/mono/metadata/appdomain.c b/src/mono/mono/metadata/appdomain.c
index fc12cc9a21675c..f0a7789108d552 100644
--- a/src/mono/mono/metadata/appdomain.c
+++ b/src/mono/mono/metadata/appdomain.c
@@ -880,9 +880,10 @@ mono_runtime_install_appctx_properties (void)
 	for (int i = 0; i < n_appctx_props; ++i) {
 		glong num_chars;
 		combined_keys [i] = g_utf8_to_utf16 (appctx_keys [i], -1, NULL, &num_chars, NULL);
-		combined_key_lengths [i] = GLONG_TO_UINT32 (num_chars);
+		// HACK: items_written from g_utf8_to_utf16 includes the null terminator unless you pass an explicit length.
+		combined_key_lengths [i] = GLONG_TO_UINT32 (num_chars ? num_chars - 1 : 0);
 		combined_values [i] = g_utf8_to_utf16 (appctx_values [i], -1, NULL, &num_chars, NULL);
-		combined_value_lengths [i] = GLONG_TO_UINT32 (num_chars);
+		combined_value_lengths [i] = GLONG_TO_UINT32 (num_chars ? num_chars - 1 : 0);
 	}
 
 	runtimeconfig_json_read_props (