diff --git a/src/ax25.c b/src/ax25.c
index c9dd7ad..f5859a9 100644
--- a/src/ax25.c
+++ b/src/ax25.c
@@ -23,6 +23,8 @@ int patty_ax25_pton(const char *callsign,
 
     enum addr_state state = ADDR_CALLSIGN;
 
+    uint8_t c = '\0';
+
     if (len == 0) {
         goto error_invalid_callsign;
     } else if (len > PATTY_AX25_ADDRSTRLEN) {
@@ -30,7 +32,7 @@ int patty_ax25_pton(const char *callsign,
     }
 
     for (i=0; i<len; i++) {
-        uint8_t c = callsign[i];
+        c = callsign[i];
 
         switch (state) {
             case ADDR_CALLSIGN:
@@ -64,6 +66,10 @@ int patty_ax25_pton(const char *callsign,
         }
     }
 
+    if (c == '-') {
+        goto error_invalid_callsign;
+    }
+
     while (o < PATTY_AX25_CALLSTRLEN) {
         addr->callsign[o++] = ' ' << 1;
     }