diff --git a/prometeu-compiler/frontends/prometeu-frontend-pbs/src/main/java/p/studio/compiler/pbs/semantics/PbsFlowTypeOps.java b/prometeu-compiler/frontends/prometeu-frontend-pbs/src/main/java/p/studio/compiler/pbs/semantics/PbsFlowTypeOps.java
index 23e8b4a2..b15e695f 100644
--- a/prometeu-compiler/frontends/prometeu-frontend-pbs/src/main/java/p/studio/compiler/pbs/semantics/PbsFlowTypeOps.java
+++ b/prometeu-compiler/frontends/prometeu-frontend-pbs/src/main/java/p/studio/compiler/pbs/semantics/PbsFlowTypeOps.java
@@ -51,7 +51,7 @@ final class PbsFlowTypeOps {
             case OPTIONAL -> compatible(actual.inner(), expected.inner());
             case RESULT -> compatible(actual.errorType(), expected.errorType()) && compatible(actual.inner(), expected.inner());
             case TUPLE -> tupleCompatible(actual, expected);
-            case UNKNOWN -> true;
+            default -> throw new IllegalStateException("Unexpected value: " + actual.kind());
         };
     }