diff --git a/src/e1cap_dump.c b/src/e1cap_dump.c
index bcb4916..4cc8538 100644
--- a/src/e1cap_dump.c
+++ b/src/e1cap_dump.c
@@ -153,11 +153,12 @@ static void handle_sc_in(struct osmo_e1cap_pkthdr *pkt, const uint8_t *data, uns
 }
 
 
-static void handle_data(struct osmo_e1cap_pkthdr *pkt, uint8_t *data, int len)
+static void handle_data(struct osmo_e1cap_pkthdr *pkt, const uint8_t *idata, int len)
 {
+	uint8_t data[len];
 	struct timeval tv;
 
-	flip_buf_bits(data, len);
+	flip_buf_bits(data, idata, len);
 #if 0
 	/* filter out all-ff/all-fe/all-7f */
 	if (all_bytes_are(0xff, data, len) ||
@@ -193,7 +194,7 @@ static void handle_data(struct osmo_e1cap_pkthdr *pkt, uint8_t *data, int len)
 	}
 }
 
-static int subch_demux_out_cb(struct subch_demux *dmx, int ch, uint8_t *data,
+static int subch_demux_out_cb(struct subch_demux *dmx, int ch, const ubit_t *data,
 			      int len, void *c)
 {
 	OSMO_ASSERT(ch == g_filter_subslot);
diff --git a/src/flip_bits.c b/src/flip_bits.c
index f33994e..6b382ed 100644
--- a/src/flip_bits.c
+++ b/src/flip_bits.c
@@ -16,14 +16,13 @@ void init_flip_bits(void)
         }
 }
 
-uint8_t *flip_buf_bits(uint8_t *buf, int len)
+uint8_t *flip_buf_bits(uint8_t *out, const uint8_t *in, int len)
 {
         int i;
-        uint8_t *start = buf;
 
         for (i = 0 ; i < len; i++) {
-                buf[i] = flip_table[(uint8_t)buf[i]];
+                out[i] = flip_table[(uint8_t)in[i]];
         }
 
-        return start;
+        return out;
 }
diff --git a/src/flip_bits.h b/src/flip_bits.h
index 319a181..249e45b 100644
--- a/src/flip_bits.h
+++ b/src/flip_bits.h
@@ -1,4 +1,4 @@
 #pragma once
 
 void init_flip_bits(void);
-uint8_t *flip_buf_bits(uint8_t * buf , int len);
+uint8_t *flip_buf_bits(uint8_t *out, const uint8_t *in, int len);