2 files changed, 25 insertions, 0 deletions
diff --git a/src/main/java/org/shedlang/compiler/typechecker/TypeChecker.java b/src/main/java/org/shedlang/compiler/typechecker/TypeChecker.java
index 58396e80..3b4df564 100644
--- a/src/main/java/org/shedlang/compiler/typechecker/TypeChecker.java
+++ b/src/main/java/org/shedlang/compiler/typechecker/TypeChecker.java
@@ -1139,6 +1139,13 @@ public class TypeChecker {
         var typedExpressionResult = typeCheckExpression(untyped.expression(), context);
         var typedExpression = typedExpressionResult.value();
         context = typedExpressionResult.context();
+
+        if (!(typedExpression.type() instanceof SumType)) {
+            throw new WhenExpressionIsNotInstanceOfSumTypeError(
+                typedExpression.type(),
+                typedExpression.source()
+            );
+        }
         
         var typedCasesResult = context.mapWithContext(
             untyped.cases(),
diff --git a/src/main/java/org/shedlang/compiler/typechecker/errors/WhenExpressionIsNotInstanceOfSumTypeError.java b/src/main/java/org/shedlang/compiler/typechecker/errors/WhenExpressionIsNotInstanceOfSumTypeError.java
new file mode 100644
index 00000000..1583c8ae
--- /dev/null
+++ b/src/main/java/org/shedlang/compiler/typechecker/errors/WhenExpressionIsNotInstanceOfSumTypeError.java
@@ -0,0 +1,18 @@
+package org.shedlang.compiler.typechecker.errors;
+
+import org.shedlang.compiler.errors.SourceError;
+import org.shedlang.compiler.sources.Source;
+import org.shedlang.compiler.types.Type;
+
+public class WhenExpressionIsNotInstanceOfSumTypeError extends SourceError {
+    private final Type actualType;
+
+    public WhenExpressionIsNotInstanceOfSumTypeError(Type actualType, Source source) {
+        super("when expression must be an instance of a sum type, but was " + actualType.describe(), source);
+        this.actualType = actualType;
+    }
+
+    public Type actualType() {
+        return actualType;
+    }
+}