diff --git a/eo-runtime/src/main/java/org/eolang/AtWithRho.java b/eo-runtime/src/main/java/org/eolang/AtWithRho.java
index bb25fffa51..30f514ace3 100644
--- a/eo-runtime/src/main/java/org/eolang/AtWithRho.java
+++ b/eo-runtime/src/main/java/org/eolang/AtWithRho.java
@@ -71,4 +71,12 @@ public Phi get() {
     public void put(final Phi phi) {
         this.origin.put(phi);
     }
+
+    /**
+     * Returns the original attribute.
+     * @return The original attribute
+     */
+    public Attr getOrigin() {
+        return this.origin;
+    }
 }
diff --git a/eo-runtime/src/main/java/org/eolang/PhDefault.java b/eo-runtime/src/main/java/org/eolang/PhDefault.java
index 4de4f93877..d05ad51c8d 100644
--- a/eo-runtime/src/main/java/org/eolang/PhDefault.java
+++ b/eo-runtime/src/main/java/org/eolang/PhDefault.java
@@ -140,7 +140,17 @@ public boolean hasRho() {
 
     @Override
     public void put(final int pos, final Phi object) {
-        this.put(this.attr(pos), object);
+        final String name = this.attr(pos);
+        final AtWithRho attribute = (AtWithRho) this.attrs.get(name);
+        if (!(attribute.getOrigin() instanceof AtVoid)) {
+            throw new ExReadOnly(
+                String.format(
+                    "There are no void attributes left, can't set the %s one",
+                    PhDefault.ordinal(pos)
+                )
+            );
+        }
+        this.put(name, object);
     }
 
     @Override
@@ -336,4 +346,34 @@ private static void debug(final String msg) {
     private static String padding() {
         return String.join("", Collections.nCopies(PhDefault.NESTING.get(), "·"));
     }
+
+    /**
+     * Returns the ordinal representation of a number (e.g., "1st", "2nd", "3rd", "4th").
+     * @param number The number to format
+     * @return The number with its ordinal suffix
+     */
+    private static String ordinal(final int number) {
+        final int lasts = number % 100;
+        final int last = number % 10;
+        final String suffix;
+        if (lasts >= 11 && lasts <= 13) {
+            suffix = "th";
+        } else {
+            switch (last) {
+                case 1:
+                    suffix = "st";
+                    break;
+                case 2:
+                    suffix = "nd";
+                    break;
+                case 3:
+                    suffix = "rd";
+                    break;
+                default:
+                    suffix = "th";
+                    break;
+            }
+        }
+        return number + suffix;
+    }
 }
diff --git a/eo-runtime/src/test/java/org/eolang/PhDefaultTest.java b/eo-runtime/src/test/java/org/eolang/PhDefaultTest.java
index 1addd5b097..8056f89ca2 100644
--- a/eo-runtime/src/test/java/org/eolang/PhDefaultTest.java
+++ b/eo-runtime/src/test/java/org/eolang/PhDefaultTest.java
@@ -23,6 +23,7 @@
  */
 package org.eolang;
 
+import EOorg.EOeolang.EOnumber;
 import com.yegor256.Together;
 import java.security.SecureRandom;
 import org.cactoos.set.SetOf;
@@ -488,6 +489,19 @@ void doesNotCalculateRandomTwice() {
         );
     }
 
+    @Test
+    void failsCorrectlyWhenTooManyAttributesPut() {
+        MatcherAssert.assertThat(
+            "the message explains what's going on",
+            Assertions.assertThrows(
+                ExAbstract.class,
+                () -> new EOnumber().put(1, new Data.ToPhi(1)),
+                "fails when trying to set attribute with too big position"
+            ).getMessage(),
+            Matchers.equalTo("There are no void attributes left, can't set the 1st one")
+        );
+    }
+
     /**
      * Rnd.
      * @since 0.1.0