diff --git a/src/iosource/pcap/functions.bif b/src/iosource/pcap/functions.bif
index 6ba880c3b1..4465510987 100644
--- a/src/iosource/pcap/functions.bif
+++ b/src/iosource/pcap/functions.bif
@@ -26,7 +26,7 @@ function precompile_pcap_filter%(id: PcapFilterID, s: string%): bool
 		// We use a vector as underlying data structure for fast
 		// lookups and limit the ID space so that that doesn't grow too
 		// large.
-		builtin_error(fmt("PCAP filter ids must remain below 100 (is %ld)", id->AsInt()));
+		builtin_error(fmt("PCAP filter ids must remain below 100 (is %" PRId64 ")", id->AsInt()));
 		return new Val(false, TYPE_BOOL);
 		}