diff --git a/subjects/isprime/README.md b/subjects/isprime/README.md
index 5da1d49f..a97c4265 100644
--- a/subjects/isprime/README.md
+++ b/subjects/isprime/README.md
@@ -6,6 +6,8 @@ Write a function that returns `true` if the `int` passed as parameter is a prime
 
 The function must be optimized in order to avoid time-outs with the tester.
 
+(We consider that only positive numbers can be prime numbers)
+
 ### Expected function
 
 ```go