diff --git a/src/classes/PMusrCanvas.cpp b/src/classes/PMusrCanvas.cpp
index cf80f93d..0d7ee761 100644
--- a/src/classes/PMusrCanvas.cpp
+++ b/src/classes/PMusrCanvas.cpp
@@ -332,7 +332,7 @@ void PMusrCanvas::UpdateParamTheoryPad()
     if (param[i].fPosErrorPresent) { // minos was used
       // calculate the arithmetic average of the pos. and neg. error
       double err;
-      err = param[i].fPosError - param[i].fStep / 2.0;
+      err = param[i].fPosError + param[i].fStep / 2.0;
       // check if the pos. and neg. error within 10%
       if ((fabs(fabs(param[i].fStep) - param[i].fPosError) < 0.1*fabs(param[i].fStep)) &&
           (fabs(fabs(param[i].fStep) - param[i].fPosError) < 0.1*param[i].fPosError)) {