diff --git a/piker/data/_sharedmem.py b/piker/data/_sharedmem.py
index 5f7fdcd0..103fa6d5 100644
--- a/piker/data/_sharedmem.py
+++ b/piker/data/_sharedmem.py
@@ -258,7 +258,7 @@ class ShmArray:
             if index < 0:
                 raise ValueError(
                     f'Array size of {self._len} was overrun during prepend.\n'
-                    'You have passed {abs(index)} too many datums.'
+                    f'You have passed {abs(index)} too many datums.'
                 )
 
         end = index + length