diff --git a/src/Network/garmin.cxx b/src/Network/garmin.cxx
index 6a0814b72..40238c1ad 100644
--- a/src/Network/garmin.cxx
+++ b/src/Network/garmin.cxx
@@ -373,7 +373,7 @@ bool FGGarmin::process() {
 	    return false;
 	}
     } else if ( get_direction() == in ) {
-	if ( length = io->readline( buf, FG_MAX_MSG_SIZE ) ) {
+	if ( (length = io->readline( buf, FG_MAX_MSG_SIZE )) > 0 ) {
 	    FG_LOG( FG_IO, FG_ALERT, "Success reading data." );
 	    if ( parse_message() ) {
 		FG_LOG( FG_IO, FG_ALERT, "Success parsing data." );
@@ -384,7 +384,7 @@ bool FGGarmin::process() {
 	    FG_LOG( FG_IO, FG_ALERT, "Error reading data." );
 	    return false;
 	}
-	if ( length = io->readline( buf, FG_MAX_MSG_SIZE ) ) {
+	if ( (length = io->readline( buf, FG_MAX_MSG_SIZE )) > 0 ) {
 	    FG_LOG( FG_IO, FG_ALERT, "Success reading data." );
 	    if ( parse_message() ) {
 		FG_LOG( FG_IO, FG_ALERT, "Success parsing data." );
diff --git a/src/Network/nmea.cxx b/src/Network/nmea.cxx
index 7eeb98827..d32d6953d 100644
--- a/src/Network/nmea.cxx
+++ b/src/Network/nmea.cxx
@@ -476,13 +476,13 @@ bool FGNMEA::process() {
 	    return false;
 	}
     } else if ( get_direction() == in ) {
-	if ( length = io->readline( buf, FG_MAX_MSG_SIZE ) ) {
+	if ( (length = io->readline( buf, FG_MAX_MSG_SIZE )) > 0 ) {
 	    parse_message();
 	} else {
 	    FG_LOG( FG_IO, FG_ALERT, "Error reading data." );
 	    return false;
 	}
-	if ( length = io->readline( buf, FG_MAX_MSG_SIZE ) ) {
+	if ( (length = io->readline( buf, FG_MAX_MSG_SIZE )) > 0 ) {
 	    parse_message();
 	} else {
 	    FG_LOG( FG_IO, FG_ALERT, "Error reading data." );