diff --git a/openbsc/src/gsm_utils.c b/openbsc/src/gsm_utils.c
index ddfd7f3..fe0b496 100644
--- a/openbsc/src/gsm_utils.c
+++ b/openbsc/src/gsm_utils.c
@@ -90,8 +90,10 @@
 			return 0;
 		else if (dbm < 5)
 			return 19;
-		else
+		else {
+			/* we are guaranteed to have (5 <= dbm < 39) */
 			return 2 + ((39 - dbm) / 2);
+		}
 		break;
 	case GSM_BAND_1800:
 		if (dbm >= 36)
@@ -100,16 +102,24 @@
 			return 30;
 		else if (dbm >= 32)
 			return 31;
-		else
+		else if (dbm == 31)
+			return 0;
+		else {
+			/* we are guaranteed to have (0 <= dbm < 31) */
 			return (30 - dbm) / 2;
+		}
 		break;
 	case GSM_BAND_1900:
 		if (dbm >= 33)
 			return 30;
 		else if (dbm >= 32)
 			return 31;
-		else
+		else if (dbm == 31)
+			return 0;
+		else {
+			/* we are guaranteed to have (0 <= dbm < 31) */
 			return (30 - dbm) / 2;
+		}
 		break;
 	}
 	return -EINVAL;