diff --git a/plugins/channeltx/modwfm/wfmmod.cpp b/plugins/channeltx/modwfm/wfmmod.cpp
index e7979ea89..cd41e8aa3 100644
--- a/plugins/channeltx/modwfm/wfmmod.cpp
+++ b/plugins/channeltx/modwfm/wfmmod.cpp
@@ -215,7 +215,7 @@ void WFMMod::pullAF(Complex& sample)
     }
 }
 
-void WFMMod::calculateLevel(Real& sample)
+void WFMMod::calculateLevel(const Real& sample)
 {
     if (m_levelCalcCount < m_levelNbSamples)
     {
diff --git a/plugins/channeltx/modwfm/wfmmod.h b/plugins/channeltx/modwfm/wfmmod.h
index af5b9140e..47282bd95 100644
--- a/plugins/channeltx/modwfm/wfmmod.h
+++ b/plugins/channeltx/modwfm/wfmmod.h
@@ -326,7 +326,7 @@ private:
 
     void apply();
     void pullAF(Complex& sample);
-    void calculateLevel(Real& sample);
+    void calculateLevel(const Real& sample);
     void openFileStream();
     void seekFileStream(int seekPercentage);
 };