diff --git a/pkl-core/src/main/java/org/pkl/core/runtime/CommandSpecParser.java b/pkl-core/src/main/java/org/pkl/core/runtime/CommandSpecParser.java
index 0c677556..b80c934a 100644
--- a/pkl-core/src/main/java/org/pkl/core/runtime/CommandSpecParser.java
+++ b/pkl-core/src/main/java/org/pkl/core/runtime/CommandSpecParser.java
@@ -32,7 +32,6 @@ import java.util.Set;
 import java.util.function.BiFunction;
 import java.util.function.Function;
 import java.util.function.Supplier;
-import java.util.stream.Collectors;
 import org.graalvm.collections.EconomicMap;
 import org.pkl.core.CommandSpec;
 import org.pkl.core.CommandSpec.Argument;
@@ -771,18 +770,18 @@ public final class CommandSpecParser {
         return true;
       } else if (typeNode
           instanceof TypeNode.UnionOfStringLiteralsTypeNode unionOfStringLiteralsTypeNode) {
-        var choices = unionOfStringLiteralsTypeNode.getStringLiterals().stream().sorted();
+        var choices = unionOfStringLiteralsTypeNode.getStringLiterals().stream().sorted().toList();
         if (each == null)
           each =
               (rawValue, workingDirUri) -> {
                 if (!unionOfStringLiteralsTypeNode.getStringLiterals().contains(rawValue)) {
-                  throw BadValue.invalidChoice(rawValue, choices.toList());
+                  throw BadValue.invalidChoice(rawValue, choices);
                 }
                 return rawValue;
               };
         if (all == null) all = this::allChooseLast;
         if (multiple == null) multiple = false;
-        if (metavar == null) metavar = "[" + choices.collect(Collectors.joining(", ")) + "]";
+        if (metavar == null) metavar = "[" + String.join(", ", choices) + "]";
         if (completionCandidates == null)
           completionCandidates = new Fixed(unionOfStringLiteralsTypeNode.getStringLiterals());
         return true;
diff --git a/pkl-core/src/test/kotlin/org/pkl/core/runtime/CommandSpecParserTest.kt b/pkl-core/src/test/kotlin/org/pkl/core/runtime/CommandSpecParserTest.kt
index bafbd485..05aaa1d1 100644
--- a/pkl-core/src/test/kotlin/org/pkl/core/runtime/CommandSpecParserTest.kt
+++ b/pkl-core/src/test/kotlin/org/pkl/core/runtime/CommandSpecParserTest.kt
@@ -787,4 +787,31 @@ class CommandSpecParserTest {
       .contains("Option `foo` with annotation `@CountedFlag` has invalid type `String`.")
     assertThat(exc.message).contains("Expected type: `Int`")
   }
+
+  @Test
+  fun `union typed option validates invalid choice without stream error`() {
+    val moduleUri =
+      writePklFile(
+        "cmd.pkl",
+        renderOptions +
+          """
+      class Options {
+        format: "json" | "yaml" | "toml"
+      }
+    """
+            .trimIndent(),
+      )
+
+    val spec = parse(moduleUri)
+    val flag = spec.options.first() as CommandSpec.Flag
+
+    assertThat(flag.metavar()).isEqualTo("[json, toml, yaml]")
+
+    val apply =
+      assertThrows<CommandSpec.Option.BadValue> {
+        flag.transformEach().apply("xml", URI("file:///tmp"))
+      }
+    assertThat(apply.message).contains("invalid choice")
+    assertThat(apply.message).contains("xml")
+  }
 }