diff --git a/esmvalcore/_main.py b/esmvalcore/_main.py
index 93e24de343..fba8c7a9ac 100755
--- a/esmvalcore/_main.py
+++ b/esmvalcore/_main.py
@@ -382,6 +382,8 @@ def run(recipe,
         logger.info("Writing program log files to:\n%s", "\n".join(log_files))
 
         cfg['skip-nonexistent'] = skip_nonexistent
+        if isinstance(diagnostics, str):
+            diagnostics = diagnostics.split(' ')
         cfg['diagnostics'] = {
             pattern if TASKSEP in pattern else pattern + TASKSEP + '*'
             for pattern in diagnostics or ()