diff --git a/src/libespeak-ng/numbers.c b/src/libespeak-ng/numbers.c
index a71e155f..afcda486 100644
--- a/src/libespeak-ng/numbers.c
+++ b/src/libespeak-ng/numbers.c
@@ -1200,14 +1200,15 @@ static int LookupNum2(Translator *tr, int value, int thousandplex, const int con
 				sprintf(ph_out, "%s%s%s%s", ph_tens, ph_and, ph_digits, ph_ordinal);
 			used_and = 1;
 		} else {
-			if ((tr->langopts.numbers & NUM_SINGLE_VOWEL) && ph_digits[0] != 0) {
+			if (tr->langopts.numbers & NUM_SINGLE_VOWEL) {
 				// remove vowel from the end of tens if units starts with a vowel (LANG=Italian)
-				ix = strlen(ph_tens) - 1;
-				if ((next_phtype = phoneme_tab[(unsigned int)(ph_digits[0])]->type) == phSTRESS)
-					next_phtype = phoneme_tab[(unsigned int)(ph_digits[1])]->type;
+				if (((ix = strlen(ph_tens)-1) >= 0) && (ph_digits[0] != 0)) {
+					if ((next_phtype = phoneme_tab[(unsigned int)(ph_digits[0])]->type) == phSTRESS)
+						next_phtype = phoneme_tab[(unsigned int)(ph_digits[1])]->type;
 
-				if ((phoneme_tab[(unsigned int)(ph_tens[ix])]->type == phVOWEL) && (next_phtype == phVOWEL))
-					ph_tens[ix] = 0;
+					if ((phoneme_tab[(unsigned int)(ph_tens[ix])]->type == phVOWEL) && (next_phtype == phVOWEL))
+						ph_tens[ix] = 0;
+				}
 			}
 
 			if ((tr->langopts.numbers2 & NUM2_ORDINAL_DROP_VOWEL) && (ph_ordinal[0] != 0)) {