diff --git a/main.c b/main.c
index 83416b5..1b99168 100644
--- a/main.c
+++ b/main.c
@@ -164,6 +164,12 @@ static void edid_modify(uint8_t* edid)
 {
 	// modify EDID to include the character indicating the firewall
 	const char firewall_indicator = '|'; // pipe/wall character to indicate the firewall
+
+	// ensure we only have up to one extension
+	if (edid[126] > 1) {
+		edid[126] = 1;
+	}
+
 	// ensure descriptor 4 is for Display name
 	if ((0 == edid[108]) && (0 == edid[109]) && (0 == edid[110]) && (0xfc == edid[111]) && (0 == edid[112])) { // ensure descriptor 4 is for Display name
 		uint8_t last_c; // position of last character
@@ -179,14 +185,15 @@ static void edid_modify(uint8_t* edid)
 			while (last_c < 126) {
 				edid[last_c++] = ' '; // insert padding space
 			}
-			// calculate new checksum
-			uint8_t checksum = 0;
-			for (uint8_t i = 0; i < 127; i++) {
-				checksum += edid[i];
-			}
-			edid[127] = (256 - checksum);
 		}
 	}
+
+	// calculate new checksum
+	uint8_t checksum = 0;
+	for (uint8_t i = 0; i < 127; i++) {
+		checksum += edid[i];
+	}
+	edid[127] = (256 - checksum);
 }
 
 void main(void)