diff --git a/python/receiver/gsm_input.py b/python/receiver/gsm_input.py
index 47edcc5..c55b823 100644
--- a/python/receiver/gsm_input.py
+++ b/python/receiver/gsm_input.py
@@ -37,7 +37,11 @@
         ##################################################
         # Blocks
         ##################################################
-        self.ppm_in = None;self.message_port_register_hier_out("ppm_in")
+        if gr.version() >= '3.7.9':
+            self.message_port_register_hier_in("ppm_in")
+        else:
+            self.message_port_register_hier_out("ppm_in")
+
         self.low_pass_filter_0_0 = filter.fir_filter_ccf(1, firdes.low_pass(
         	1, samp_rate_out, 125e3, 5e3, firdes.WIN_HAMMING, 6.76))
         self.gsm_clock_offset_corrector_0 = grgsm.clock_offset_corrector(