diff --git a/src/main/java/org/cactoos/io/NotNullInput.java b/src/main/java/org/cactoos/io/NotNullInput.java
index 4dbf4983b4..8d481c2537 100644
--- a/src/main/java/org/cactoos/io/NotNullInput.java
+++ b/src/main/java/org/cactoos/io/NotNullInput.java
@@ -54,7 +54,7 @@ public NotNullInput(final Input input) {
     @Override
     public InputStream stream() throws IOException {
         if (this.origin == null) {
-            throw new IOException("invalid input (null)");
+            throw new IOException("NULL instead of a valid input");
         }
         return this.origin.stream();
     }
diff --git a/src/main/java/org/cactoos/io/NotNullOutput.java b/src/main/java/org/cactoos/io/NotNullOutput.java
index b79c46ed14..82b072e2fd 100644
--- a/src/main/java/org/cactoos/io/NotNullOutput.java
+++ b/src/main/java/org/cactoos/io/NotNullOutput.java
@@ -54,7 +54,7 @@ public NotNullOutput(final Output output) {
     @Override
     public OutputStream stream() throws IOException {
         if (this.origin == null) {
-            throw new IOException("invalid output (null)");
+            throw new IOException("NULL instead of a valid output");
         }
         return this.origin.stream();
     }