diff --git a/src/conv.c b/src/conv.c
index cf820a3..70bdffb 100644
--- a/src/conv.c
+++ b/src/conv.c
@@ -389,9 +389,12 @@
 			m = 1 << (code->N - 1);		/* mask for 'out' bit selection */
 
 			for (j=0; j<code->N; j++) {
-				ov = (out & m) ? -127 : 127; /* sbit_t value for it */
-				e = ((int)in_sym[j]) - ov;   /* raw error for this bit */
-				nae += (e * e) >> 9;         /* acc the squared/scaled value */
+				int is = (int)in_sym[j];
+				if (is) {
+					ov = (out & m) ? -127 : 127; /* sbit_t value for it */
+					e = is - ov;                 /* raw error for this bit */
+					nae += (e * e) >> 9;         /* acc the squared/scaled value */
+				}
 				m >>= 1;                     /* next mask bit */
 			}