diff --git a/hardware/common/serial.c b/hardware/common/serial.c
index 5a1d21af..bc837a38 100644
--- a/hardware/common/serial.c
+++ b/hardware/common/serial.c
@@ -603,9 +603,9 @@ SR_PRIV int serial_set_paramstr(int fd, const char *paramstr)
 }
 
 SR_PRIV int serial_readline(int fd, char **buf, int *buflen,
-			    uint64_t timeout_ms)
+			    gint64 timeout_ms)
 {
-	uint64_t start;
+	gint64 start;
 	int maxlen, len;
 
 	timeout_ms *= 1000;
diff --git a/libsigrok-internal.h b/libsigrok-internal.h
index 4e46805a..c63dcd46 100644
--- a/libsigrok-internal.h
+++ b/libsigrok-internal.h
@@ -136,7 +136,7 @@ SR_PRIV int serial_set_params(int fd, int baudrate, int bits, int parity,
 			      int stopbits, int flowcontrol);
 SR_PRIV int serial_set_paramstr(int fd, const char *paramstr);
 SR_PRIV int serial_readline(int fd, char **buf, int *buflen,
-			    uint64_t timeout_ms);
+			    gint64 timeout_ms);
 
 /*--- hardware/common/ezusb.c -----------------------------------------------*/