diff --git a/math/number-theory/is-prime.hpp b/math/number-theory/is-prime.hpp
index 0552a0c1..2edfa553 100644
--- a/math/number-theory/is-prime.hpp
+++ b/math/number-theory/is-prime.hpp
@@ -3,6 +3,7 @@
 #include <cstdint>
 
 bool is_prime(std::int64_t x) {
+  if (x < 2) return false;
   for (std::int64_t i = 2; i * i <= x; i++) {
     if (x % i == 0) return false;
   }
diff --git a/test/unittest/is-prime.test.cpp b/test/unittest/is-prime.test.cpp
new file mode 100644
index 00000000..a445c0d6
--- /dev/null
+++ b/test/unittest/is-prime.test.cpp
@@ -0,0 +1,15 @@
+// competitive-verifier: STANDALONE
+
+#include "../../template/template.hpp"
+
+#include "../../math/number-theory/is-prime.hpp"
+
+int main() {
+  assert(!is_prime(0));
+  assert(!is_prime(1));
+  assert(is_prime(2));
+  assert(is_prime(3));
+  assert(!is_prime(4));
+  assert(is_prime(999999937));
+  assert(!is_prime(1000000000));
+}