diff --git a/libraries/AP_Math/AP_Math.cpp b/libraries/AP_Math/AP_Math.cpp
index 86bff17b5b..0322965023 100644
--- a/libraries/AP_Math/AP_Math.cpp
+++ b/libraries/AP_Math/AP_Math.cpp
@@ -118,7 +118,7 @@ float linear_interpolate(float low_output, float high_output,
  * alpha range: [0,1] min to max expo
  * input range: [-1,1]
  */
-constexpr float expo_curve(float alpha, float x)
+float expo_curve(float alpha, float x)
 {
     return (1.0f - alpha) * x + alpha * x * x * x;
 }
diff --git a/libraries/AP_Math/AP_Math.h b/libraries/AP_Math/AP_Math.h
index c80d9d295d..644f5964ab 100644
--- a/libraries/AP_Math/AP_Math.h
+++ b/libraries/AP_Math/AP_Math.h
@@ -271,7 +271,7 @@ float linear_interpolate(float low_output, float high_output,
  * alpha range: [0,1] min to max expo
  * input range: [-1,1]
  */
-constexpr float expo_curve(float alpha, float input);
+float expo_curve(float alpha, float input);
 
 /* throttle curve generator
  * thr_mid: output at mid stick