diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index d451875f526..2550e8bd8bc 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -4100,7 +4100,7 @@ namespace ts {
                     flags: flags || NodeBuilderFlags.None,
                     // If no full tracker is provided, fake up a dummy one with a basic limited-functionality moduleResolverHost
                     tracker: tracker && tracker.trackSymbol ? tracker : { trackSymbol: noop, moduleResolverHost: flags! & NodeBuilderFlags.DoNotIncludeSymbolChain ? {
-                        getCommonSourceDirectory: (host as Program).getCommonSourceDirectory ? () => (host as Program).getCommonSourceDirectory() : () => "",
+                        getCommonSourceDirectory: !!(host as Program).getCommonSourceDirectory ? () => (host as Program).getCommonSourceDirectory() : () => "",
                         getSourceFiles: () => host.getSourceFiles(),
                         getCurrentDirectory: () => host.getCurrentDirectory(),
                         getProbableSymlinks: maybeBind(host, host.getProbableSymlinks),