diff --git a/pymzml/spec.py b/pymzml/spec.py
index a1ddb61c..be4ac8c0 100755
--- a/pymzml/spec.py
+++ b/pymzml/spec.py
@@ -502,7 +502,11 @@ def __add__(self, other_spec):
         if self._peak_dict["reprofiled"] is None:
             reprofiled = self._reprofile_Peaks()
             self.set_peaks(reprofiled, "reprofiled")
-        for mz, i in other_spec.peaks("reprofiled"):
+        if other_spec._peak_dict['reprofiled'] is None:
+            reprofiled = other_spec._reprofile_Peaks()
+            other_spec.set_peaks(reprofiled, "reprofiled")
+        # access peak dict directly to avoid casting to array
+        for mz, i in other_spec._peak_dict['reprofiled'].items():
             self._peak_dict["reprofiled"][mz] += i
         return self