diff --git a/be-js/src/commonMain/kotlin/lang/temper/be/js/RunJs.kt b/be-js/src/commonMain/kotlin/lang/temper/be/js/RunJs.kt
index 97871306..50a682d2 100644
--- a/be-js/src/commonMain/kotlin/lang/temper/be/js/RunJs.kt
+++ b/be-js/src/commonMain/kotlin/lang/temper/be/js/RunJs.kt
@@ -65,8 +65,9 @@ internal fun runJsBestEffort(
     when (request) {
         is RunBackendSpecificCompilationStepRequest -> error("$request")
         is RunLibraryRequest -> {
+            // Null for a library the build does not know, which the metadata check below reports.
             mainLibraryConfig = dependencies.libraryConfigurations
-                .byLibraryName.getValue(request.libraryName)
+                .byLibraryName[request.libraryName]
             val jsLibraryName = dependencies.metadata[request.libraryName, JsMetadataKey.JsLibraryName]
                 ?: return listOf(
                     ToolchainResult(result = RFailure(CliFailure("No JS library name for $request"))),
diff --git a/cli/src/test/kotlin/lang/temper/cli/RunTest.kt b/cli/src/test/kotlin/lang/temper/cli/RunTest.kt
index 9b192c1b..e1f2b59f 100644
--- a/cli/src/test/kotlin/lang/temper/cli/RunTest.kt
+++ b/cli/src/test/kotlin/lang/temper/cli/RunTest.kt
@@ -31,6 +31,7 @@ import kotlin.test.Ignore
 import kotlin.test.Test
 import kotlin.test.assertContains
 import kotlin.test.assertEquals
+import kotlin.test.assertFalse
 import kotlin.test.assertTrue
 
 class RunTest {
@@ -106,6 +107,26 @@ class RunTest {
     @Test
     fun runJsBackend() = runRunTest("run", JsBackend.Factory.backendId)
 
+    @Test
+    fun runJsBackendUnknownLibrary() {
+        val path = resourcePath("/run/input")
+        // Clean up in case someone ran a build in the source tree.
+        removeDirRecursive(path.resolve(TEMPER_OUT_NAME))
+        runWithTemporaryDirCopyOf("RunJsBackendUnknownLibrary", path) { tempDir ->
+            val result = doRun(
+                RunTask(
+                    backends = setOf(JsBackend.Factory.backendId),
+                    request = RunLibraryRequest(
+                        DashedIdentifier.from("no-such-library")!!,
+                        taskName = "RunJsBackendUnknownLibrary",
+                    ),
+                ),
+                workRoot = tempDir,
+            )
+            assertFalse(result.ok)
+        }
+    }
+
     @Ignore("Reexamine how import strategy and run tasks affect running only cherry-picked modules")
     @Test
     fun runJsBackendTop() = runRunTest("runTopLevel", JsBackend.Factory.backendId)