diff --git a/src/ctbutterfly/ctbutterfly.cpp b/src/ctbutterfly/ctbutterfly.cpp
index 1ea3881..e4cb59d 100644
--- a/src/ctbutterfly/ctbutterfly.cpp
+++ b/src/ctbutterfly/ctbutterfly.cpp
@@ -674,7 +674,13 @@ void ctbutterfly::gaussian_error_propagation(GModels& models)
 
     // Write header into logger
     log_header1(TERSE, "Generate butterfly");
-    
+ 
+    // Initialise result arrays
+    m_energies.clear();
+    m_intensities.clear();
+    m_min_intensities.clear();
+    m_max_intensities.clear();
+   
     // Initialise dummy time to evaluate spectral model
     GTime time = GTime();
 
diff --git a/test/test_ctbutterfly.py b/test/test_ctbutterfly.py
index ba7fe16..0e7562a 100755
--- a/test/test_ctbutterfly.py
+++ b/test/test_ctbutterfly.py
@@ -168,6 +168,9 @@ class Test(test):
         cpy_butterfly['outfile'] = 'ctbutterfly_py3.dat'
         cpy_butterfly['logfile'] = 'ctbutterfly_py3.log'
         cpy_butterfly['chatter'] = 4
+
+        # Execute ctbutterfly tool
+        cpy_butterfly.logFileOpen()   # Make sure we get a log file
         cpy_butterfly.execute()
 
         # Check result file