diff --git a/src/libthermo/nnparams.c b/src/libthermo/nnparams.c
index 9400320..71e71d5 100644
--- a/src/libthermo/nnparams.c
+++ b/src/libthermo/nnparams.c
@@ -487,26 +487,28 @@ void nparam_CleanSeq (char* inseq, char* outseq, int len)
 	if (len != 0)
 		seqlen = len;
 
-	for (i = 0, j = 0; i < seqlen; i++)
+	outseq[0]='x';
+
+	for (i = 0, j = 0; i < seqlen && outseq[0]; i++,j++)
 	{
 		switch (inseq[i])
 		{
 		case 'a':
 		case '\0':
 		case 'A':
-			outseq[j++] = 'A'; break;
+			outseq[j] = 'A'; break;
 		case 'c':
 		case '\1':
 		case 'C':
-			outseq[j++] = 'C'; break;
+			outseq[j] = 'C'; break;
 		case 'g':
 		case '\2':
 		case 'G':
-			outseq[j++] = 'G'; break;
+			outseq[j] = 'G'; break;
 		case 't':
 		case '\3':
 		case 'T':
-			outseq[j++] = 'T'; break;
+			outseq[j] = 'T'; break;
 		default:
 			outseq[0]=0;
 		}