diff --git a/scsh/fdports1.c b/scsh/fdports1.c
index 1230fda..0516115 100644
--- a/scsh/fdports1.c
+++ b/scsh/fdports1.c
@@ -395,6 +395,7 @@ int read_fdport_substring(scheme_value buf, int start, int end, scheme_value dat
 	if( len > 0 ) {
 	    char *p = StrByte(buf,start);
 	    *p++ = EXTRACT_CHAR(peek);
+            *PortData_Peek(data) = SCHFALSE;
 	    return 1 + fread(p, 1, MIN(len-1, fbufcount(f)), f);
 	    }
 	else return 0;