diff --git a/src/main/java/net/fabricmc/loom/configuration/mods/AccessWidenerUtils.java b/src/main/java/net/fabricmc/loom/configuration/mods/AccessWidenerUtils.java
index 915404c1..725e4fb1 100644
--- a/src/main/java/net/fabricmc/loom/configuration/mods/AccessWidenerUtils.java
+++ b/src/main/java/net/fabricmc/loom/configuration/mods/AccessWidenerUtils.java
@@ -57,6 +57,10 @@ public class AccessWidenerUtils {
 	}
 
 	public static AccessWidenerData readAccessWidenerData(Path inputJar) throws IOException {
+		if (!FabricModJsonFactory.isModJar(inputJar)) {
+			return null;
+		}
+
 		final FabricModJson fabricModJson = FabricModJsonFactory.createFromZip(inputJar);
 		final List<String> classTweakers = List.copyOf(fabricModJson.getClassTweakers().keySet());