diff --git a/homology.tex b/homology.tex
index 7a38c83ef..973aa9ff9 100644
--- a/homology.tex
+++ b/homology.tex
@@ -1033,7 +1033,7 @@ \section{Extensions}
 \begin{definition}
 \label{definition-extension}
 Let $\mathcal{A}$ be an abelian category.
-Let $A, C \in \Ob(\mathcal{A})$.
+Let $A, B \in \Ob(\mathcal{A})$.
 An {\it extension $E$ of $B$ by $A$} is a short
 exact sequence
 $$