diff --git a/backend/gs1.c b/backend/gs1.c
index e567b5f1..861eab2e 100644
--- a/backend/gs1.c
+++ b/backend/gs1.c
@@ -65,17 +65,17 @@ int gs1_verify(struct zint_symbol *symbol, unsigned char source[], const unsigne
 	for(i = 0; i < src_len; i++) {
 		if(source[i] >=128) {
 			strcpy(symbol->errtxt, "Extended ASCII characters are not supported by GS1");
-			return ERROR_INVALID_DATA;
+			return ZERROR_INVALID_DATA;
 		}
 		if(source[i] < 32) {
 			strcpy(symbol->errtxt, "Control characters are not supported by GS1");
-			return ERROR_INVALID_DATA;
+			return ZERROR_INVALID_DATA;
 		}
 	}
 	
 	if(source[0] != '[') {
 		strcpy(symbol->errtxt, "Data does not start with an AI");
-		return ERROR_INVALID_DATA;
+		return ZERROR_INVALID_DATA;
 	}
 	
 	/* Check the position of the brackets */
@@ -104,31 +104,31 @@ int gs1_verify(struct zint_symbol *symbol, unsigned char source[], const unsigne
 	if(bracket_level != 0) {
 		/* Not all brackets are closed */
 		strcpy(symbol->errtxt, "Malformed AI in input data (brackets don\'t match)");
-		return ERROR_INVALID_DATA;
+		return ZERROR_INVALID_DATA;
 	}
 	
 	if(max_bracket_level > 1) {
 		/* Nested brackets */
 		strcpy(symbol->errtxt, "Found nested brackets in input data");
-		return ERROR_INVALID_DATA;
+		return ZERROR_INVALID_DATA;
 	}
 	
 	if(max_ai_length > 4) {
 		/* AI is too long */
 		strcpy(symbol->errtxt, "Invalid AI in input data (AI too long)");
-		return ERROR_INVALID_DATA;
+		return ZERROR_INVALID_DATA;
 	}
 	
 	if(min_ai_length <= 1) {
 		/* AI is too short */
 		strcpy(symbol->errtxt, "Invalid AI in input data (AI too short)");
-		return ERROR_INVALID_DATA;
+		return ZERROR_INVALID_DATA;
 	}
 	
 	if(ai_latch == 1) {
 		/* Non-numeric data in AI */
 		strcpy(symbol->errtxt, "Invalid AI in input data (non-numeric characters in AI)");
-		return ERROR_INVALID_DATA;
+		return ZERROR_INVALID_DATA;
 	}
 	
 	ai_count = 0;
@@ -161,7 +161,7 @@ int gs1_verify(struct zint_symbol *symbol, unsigned char source[], const unsigne
 		if(data_length[i] == 0) {
 			/* No data for given AI */
 			strcpy(symbol->errtxt, "Empty data field in input data");
-			return ERROR_INVALID_DATA;
+			return ZERROR_INVALID_DATA;
 		}
 	}
 	
@@ -170,48 +170,68 @@ int gs1_verify(struct zint_symbol *symbol, unsigned char source[], const unsigne
 	for(i = 0; i < ai_count; i++) {
 		switch (ai_value[i]) {
 			case 0: if(data_length[i] != 18) { error_latch = 1; } break;
-			case 1: if(data_length[i] != 14) { error_latch = 1; } break;
-			case 2: if(data_length[i] != 14) { error_latch = 1; } break;
+			case 1:
+			case 2:
 			case 3: if(data_length[i] != 14) { error_latch = 1; } break;
 			case 4: if(data_length[i] != 16) { error_latch = 1; } break;
-			case 11: if(data_length[i] != 6) { error_latch = 1; } break;
-			case 12: if(data_length[i] != 6) { error_latch = 1; } break;
-			case 13: if(data_length[i] != 6) { error_latch = 1; } break;
-			case 14: if(data_length[i] != 6) { error_latch = 1; } break;
-			case 15: if(data_length[i] != 6) { error_latch = 1; } break;
-			case 16: if(data_length[i] != 6) { error_latch = 1; } break;
-			case 17: if(data_length[i] != 6) { error_latch = 1; } break;
-			case 18: if(data_length[i] != 6) { error_latch = 1; } break;
+			case 11:
+			case 12:
+			case 13:
+			case 14:
+			case 15:
+			case 16:
+			case 17:
+			case 18:
 			case 19: if(data_length[i] != 6) { error_latch = 1; } break;
 			case 20: if(data_length[i] != 2) { error_latch = 1; } break;
-			case 23: error_latch = 2; break;
-			case 24: error_latch = 2; break;
-			case 25: error_latch = 2; break;
-			case 39: error_latch = 2; break;
-			case 40: error_latch = 2; break;
-			case 41: error_latch = 2; break;
-			case 42: error_latch = 2; break;
-			case 70: error_latch = 2; break;
-			case 80: error_latch = 2; break;
+			case 23:
+			case 24:
+			case 25:
+			case 39:
+			case 40:
+			case 41:
+			case 42:
+			case 70:
+			case 80:
 			case 81: error_latch = 2; break;
 		}
-		if((ai_value[i] >= 100) && (ai_value[i] <= 179)) { error_latch = 2; }
-		if((ai_value[i] >= 1000) && (ai_value[i] <= 1799)) { error_latch = 2; }
-		if((ai_value[i] >= 200) && (ai_value[i] <= 229)) { error_latch = 2; }
-		if((ai_value[i] >= 2000) && (ai_value[i] <= 2299)) { error_latch = 2; }
-		if((ai_value[i] >= 300) && (ai_value[i] <= 309)) { error_latch = 2; }
-		if((ai_value[i] >= 3000) && (ai_value[i] <= 3099)) { error_latch = 2; }
-		if((ai_value[i] >= 31) && (ai_value[i] <= 36)) { error_latch = 2; }
-		if((ai_value[i] >= 310) && (ai_value[i] <= 369)) { error_latch = 2; }
-		if((ai_value[i] >= 3100) && (ai_value[i] <= 3699)) { if(data_length[i] != 6) { error_latch = 1; } }
-		if((ai_value[i] >= 370) && (ai_value[i] <= 379)) { error_latch = 2; }
-		if((ai_value[i] >= 3700) && (ai_value[i] <= 3799)) { error_latch = 2; }
-		if((ai_value[i] >= 410) && (ai_value[i] <= 415)) { if(data_length[i] != 13) { error_latch = 1; } }
-		if((ai_value[i] >= 4100) && (ai_value[i] <= 4199)) { error_latch = 2; }
-		if((ai_value[i] >= 700) && (ai_value[i] <= 703)) { error_latch = 2; }
-		if((ai_value[i] >= 800) && (ai_value[i] <= 810)) { error_latch = 2; }
-		if((ai_value[i] >= 900) && (ai_value[i] <= 999)) { error_latch = 2; }
-		if((ai_value[i] >= 9000) && (ai_value[i] <= 9999)) { error_latch = 2; }
+		if(
+			((ai_value[i] >= 100) && (ai_value[i] <= 179))
+			|| ((ai_value[i] >= 1000) && (ai_value[i] <= 1799))
+			|| ((ai_value[i] >= 200) && (ai_value[i] <= 229))
+			|| ((ai_value[i] >= 2000) && (ai_value[i] <= 2299))
+			|| ((ai_value[i] >= 300) && (ai_value[i] <= 309))
+			|| ((ai_value[i] >= 3000) && (ai_value[i] <= 3099))
+			|| ((ai_value[i] >= 31) && (ai_value[i] <= 36))
+			|| ((ai_value[i] >= 310) && (ai_value[i] <= 369))
+		) {
+			error_latch = 2;
+		}
+		if((ai_value[i] >= 3100) && (ai_value[i] <= 3699)) {
+			if(data_length[i] != 6) {
+				error_latch = 1;
+			}
+		}
+		if(
+			((ai_value[i] >= 370) && (ai_value[i] <= 379))
+			|| ((ai_value[i] >= 3700) && (ai_value[i] <= 3799))
+		) { 
+			error_latch = 2; 
+		}
+		if((ai_value[i] >= 410) && (ai_value[i] <= 415)) {
+			if(data_length[i] != 13) { 
+				error_latch = 1; 
+			} 
+		}
+		if(
+			((ai_value[i] >= 4100) && (ai_value[i] <= 4199))
+			|| ((ai_value[i] >= 700) && (ai_value[i] <= 703))
+			|| ((ai_value[i] >= 800) && (ai_value[i] <= 810))
+			|| ((ai_value[i] >= 900) && (ai_value[i] <= 999))
+			|| ((ai_value[i] >= 9000) && (ai_value[i] <= 9999))
+		) {
+			error_latch = 2; 
+		}
 		if((error_latch < 4) && (error_latch > 0)) {
 			/* error has just been detected: capture AI */
 			itostr(ai_string, ai_value[i]);
@@ -222,13 +242,13 @@ int gs1_verify(struct zint_symbol *symbol, unsigned char source[], const unsigne
 	if(error_latch == 5) {
 		strcpy(symbol->errtxt, "Invalid data length for AI ");
 		concat(symbol->errtxt, ai_string);
-		return ERROR_INVALID_DATA;
+		return ZERROR_INVALID_DATA;
 	}
 	
 	if(error_latch == 6) {
 		strcpy(symbol->errtxt, "Invalid AI value ");
 		concat(symbol->errtxt, ai_string);
-		return ERROR_INVALID_DATA;
+		return ZERROR_INVALID_DATA;
 	}
 
 	/* Resolve AI data - put resulting string in 'reduced' */
@@ -251,11 +271,15 @@ int gs1_verify(struct zint_symbol *symbol, unsigned char source[], const unsigne
 			ai_latch = 0;
 			/* The following values from "GS-1 General Specification version 8.0 issue 2, May 2008"
 			figure 5.4.8.2.1 - 1 "Element Strings with Pre-Defined Length Using Application Identifiers" */
-			if((last_ai >= 0) && (last_ai <= 4)) { ai_latch = 1; }
-			if((last_ai >= 11) && (last_ai <= 20)) { ai_latch = 1; }
-			if(last_ai == 23) { ai_latch = 1; } /* legacy support - see 5.3.8.2.2 */
-			if((last_ai >= 31) && (last_ai <= 36)) { ai_latch = 1; }
-			if(last_ai == 41) { ai_latch = 1; }
+			if(
+				((last_ai >= 0) && (last_ai <= 4))
+				|| ((last_ai >= 11) && (last_ai <= 20))
+				|| (last_ai == 23) /* legacy support - see 5.3.8.2.2 */
+				|| ((last_ai >= 31) && (last_ai <= 36))
+				|| (last_ai == 41)
+			) {
+				ai_latch = 1; 
+			}
 		}
 		/* The ']' character is simply dropped from the input */
 	}
@@ -283,5 +307,5 @@ int ugs1_verify(struct zint_symbol *symbol, unsigned char source[], const unsign
 		return 0;
 	}
 	strcpy(symbol->errtxt, "ugs1_verify overflow");
-	return ERROR_INVALID_DATA;
+	return ZERROR_INVALID_DATA;
 }