--- a/src/thal.c
+++ b/src/thal.c
@@ -426,12 +426,12 @@
 	       "Illegal type");
    o->align_end_1 = -1;
    o->align_end_2 = -1;
-   if ('\0' == oligo_f) {
+   if ('\0' == oligo_f[0]) {
       strcpy(o->msg, "Empty first sequence");
       o->temp = 0.0;
       return;
    }
-   if ('\0' == oligo_r) {
+   if ('\0' == oligo_r[0]) {
       strcpy(o->msg, "Empty second sequence");
       o->temp = 0.0;
       return;