diff --git a/doc/src/dihedral_nharmonic.rst b/doc/src/dihedral_nharmonic.rst
index d4ba345bf4..dffdb8ee96 100644
--- a/doc/src/dihedral_nharmonic.rst
+++ b/doc/src/dihedral_nharmonic.rst
@@ -28,7 +28,7 @@ The *nharmonic* dihedral style uses the potential:
 
 .. math::
 
-   E = \sum_{n=1,n} A_n  \cos^{n-1}(\phi)
+   E = \sum_{i=1,n} A_i  \cos^{i-1}(\phi)
 
 The following coefficients must be defined for each dihedral type via the
 :doc:`dihedral_coeff <dihedral_coeff>` command as in the example above, or in