diff --git a/Source/cmGlobalGenerator.cxx b/Source/cmGlobalGenerator.cxx
index bf155851c2..5d46d2a166 100644
--- a/Source/cmGlobalGenerator.cxx
+++ b/Source/cmGlobalGenerator.cxx
@@ -3389,9 +3389,12 @@ cmValue cmGlobalGenerator::GetDebuggerWorkingDirectory(
 }
 
 cmGlobalGenerator::TargetDependSet const&
-cmGlobalGenerator::GetTargetDirectDepends(cmGeneratorTarget const* target)
+cmGlobalGenerator::GetTargetDirectDepends(
+  cmGeneratorTarget const* target) const
 {
-  return this->TargetDependencies[target];
+  auto i = this->TargetDependencies.find(target);
+  assert(i != this->TargetDependencies.end());
+  return i->second;
 }
 
 bool cmGlobalGenerator::TargetOrderIndexLess(cmGeneratorTarget const* l,
diff --git a/Source/cmGlobalGenerator.h b/Source/cmGlobalGenerator.h
index e7b252859d..7d59d9a8bb 100644
--- a/Source/cmGlobalGenerator.h
+++ b/Source/cmGlobalGenerator.h
@@ -525,7 +525,7 @@ public:
   // what targets does the specified target depend on directly
   // via a target_link_libraries or add_dependencies
   TargetDependSet const& GetTargetDirectDepends(
-    cmGeneratorTarget const* target);
+    cmGeneratorTarget const* target) const;
 
   // Return true if target 'l' occurs before 'r' in a global ordering
   // of targets that respects inter-target dependencies.