diff --git a/src/common/parser/ApcParser.test.ts b/src/common/parser/ApcParser.test.ts
index b27b40039..563ddbeab 100644
--- a/src/common/parser/ApcParser.test.ts
+++ b/src/common/parser/ApcParser.test.ts
@@ -21,7 +21,7 @@ function identifier(id: IFunctionIdentifier): number {
       throw new Error('only one byte as prefix supported');
     }
     res = id.prefix.charCodeAt(0);
-    if (res && 0x3c > res || res > 0x3f) {
+    if (res < 0x3c || res > 0x3f) {
       throw new Error('prefix must be in range 0x3c .. 0x3f');
     }
   }
diff --git a/src/common/parser/DcsParser.test.ts b/src/common/parser/DcsParser.test.ts
index 459bc8961..5fa7b9416 100644
--- a/src/common/parser/DcsParser.test.ts
+++ b/src/common/parser/DcsParser.test.ts
@@ -22,7 +22,7 @@ function identifier(id: IFunctionIdentifier): number {
       throw new Error('only one byte as prefix supported');
     }
     res = id.prefix.charCodeAt(0);
-    if (res && 0x3c > res || res > 0x3f) {
+    if (res < 0x3c || res > 0x3f) {
       throw new Error('prefix must be in range 0x3c .. 0x3f');
     }
   }
diff --git a/src/common/parser/EscapeSequenceParser.ts b/src/common/parser/EscapeSequenceParser.ts
index a0c97b610..c12ca3446 100644
--- a/src/common/parser/EscapeSequenceParser.ts
+++ b/src/common/parser/EscapeSequenceParser.ts
@@ -342,7 +342,7 @@ export class EscapeSequenceParser extends Disposable implements IEscapeSequenceP
         throw new Error('only one byte as prefix supported');
       }
       res = id.prefix.charCodeAt(0);
-      if (res && 0x3c > res || res > 0x3f) {
+      if (res < 0x3c || res > 0x3f) {
         throw new Error('prefix must be in range 0x3c .. 0x3f');
       }
     }