diff --git a/doc/src/fix_rhok.txt b/doc/src/fix_rhok.txt
index 162d51e67fb0a71bf507cba9e88b552bdab75a21..8d3e5dcb1704e68b09758e7f1ac92622c896b0ae 100644
--- a/doc/src/fix_rhok.txt
+++ b/doc/src/fix_rhok.txt
@@ -8,7 +8,7 @@
 
 fix rhok command :h3
 
-fix ID group-ID rhok nx ny nz k a :pre
+fix ID group-ID rhok nx ny nz K a :pre
 
 ID, group-ID are documented in "fix"_fix.html command
 nx, ny, nz = k-vektor of collective density field