diff --git a/wiretap/file_access.c b/wiretap/file_access.c
index 5a49a38fd2..3875ffbd2b 100644
--- a/wiretap/file_access.c
+++ b/wiretap/file_access.c
@@ -1139,29 +1139,6 @@ fail:
 	return NULL;
 
 success:
-	if ((wth->file_type_subtype == WTAP_FILE_TYPE_SUBTYPE_PCAP) ||
-		(wth->file_type_subtype == WTAP_FILE_TYPE_SUBTYPE_PCAP_NSEC)) {
-
-		wtap_block_t descr = wtap_block_create(WTAP_BLOCK_IF_DESCR);
-		wtapng_if_descr_mandatory_t* descr_mand = (wtapng_if_descr_mandatory_t*)wtap_block_get_mandatory_data(descr);
-
-		descr_mand->wtap_encap = wth->file_encap;
-		if (wth->file_type_subtype == WTAP_FILE_TYPE_SUBTYPE_PCAP_NSEC) {
-			descr_mand->time_units_per_second = 1000000000; /* nanosecond resolution */
-			wtap_block_add_uint8_option(descr, OPT_IDB_TSRESOL, 9);
-			descr_mand->tsprecision = WTAP_TSPREC_NSEC;
-		} else {
-			descr_mand->time_units_per_second = 1000000; /* default microsecond resolution */
-			/* No need to add an option, this is the default */
-			descr_mand->tsprecision = WTAP_TSPREC_USEC;
-		}
-		descr_mand->snap_len = wth->snapshot_length;
-
-		descr_mand->num_stat_entries = 0;          /* Number of ISB:s */
-		descr_mand->interface_statistics = NULL;
-		wtap_add_idb(wth, descr);
-
-	}
 	return wth;
 }
 
diff --git a/wiretap/libpcap.c b/wiretap/libpcap.c
index 5e3dbf07cc..ac4c2f4dd6 100644
--- a/wiretap/libpcap.c
+++ b/wiretap/libpcap.c
@@ -521,6 +521,47 @@ done:
 		/*Reset the ERF interface lookup table*/
 		libpcap->encap_priv = erf_priv_create();
 	}
+
+	/*
+	 * Add an IDB; we don't know how many interfaces were involved,
+	 * so we just say one interface, about which we only know
+	 * the link-layer type, snapshot length, and time stamp
+	 * resolution.
+	 *
+	 * This allows nanosecond pcap files to be correctly
+	 * converted to pcapng files.
+	 *
+	 * XXX - this will be a bit weird if you're trying to convert
+	 * a LINKTYPE_ERF pcap file to a pcapng file; it'll have a
+	 * placeholder interface added here, *plus* interfaces
+	 * added from the ERF records.  Ideally, at some point in
+	 * the future, libpcap will have a more pcapng-friendly API
+	 * for capturing, and the DAG capture code will use it, so that
+	 * if you're capturing on more than one interface, they'll all
+	 * get regular IDBs, with no need for the placeholder.
+	 *
+	 * XXX - yes, adding at least one IDB should be done for *all*
+	 * file types.
+	 */
+	wtap_block_t descr = wtap_block_create(WTAP_BLOCK_IF_DESCR);
+	wtapng_if_descr_mandatory_t* descr_mand = (wtapng_if_descr_mandatory_t*)wtap_block_get_mandatory_data(descr);
+
+	descr_mand->wtap_encap = wth->file_encap;
+	if (wth->file_type_subtype == WTAP_FILE_TYPE_SUBTYPE_PCAP_NSEC) {
+		descr_mand->time_units_per_second = 1000000000; /* nanosecond resolution */
+		wtap_block_add_uint8_option(descr, OPT_IDB_TSRESOL, 9);
+		descr_mand->tsprecision = WTAP_TSPREC_NSEC;
+	} else {
+		descr_mand->time_units_per_second = 1000000; /* default microsecond resolution */
+		/* No need to add an option, this is the default */
+		descr_mand->tsprecision = WTAP_TSPREC_USEC;
+	}
+	descr_mand->snap_len = wth->snapshot_length;
+
+	descr_mand->num_stat_entries = 0;          /* Number of ISB:s */
+	descr_mand->interface_statistics = NULL;
+	wtap_add_idb(wth, descr);
+
 	return WTAP_OPEN_MINE;
 }