diff --git a/lambda-calculus.html.markdown b/lambda-calculus.html.markdown
index 53a7a7cd..958dd746 100644
--- a/lambda-calculus.html.markdown
+++ b/lambda-calculus.html.markdown
@@ -131,7 +131,7 @@ We can convert an expression in the lambda calculus to an expression
 in the SKI combinator calculus:
 
 1. `λx.x = I`
-2. `λx.c = Kc`
+2. `λx.c = Kc` provided that `x` does not occur free in `c`
 3. `λx.(y z) = S (λx.y) (λx.z)`
 
 Take the church number 2 for example: