diff --git a/spock-core/src/main/java/org/spockframework/runtime/extension/builtin/RepeatUntilFailureExtension.java b/spock-core/src/main/java/org/spockframework/runtime/extension/builtin/RepeatUntilFailureExtension.java
index 52af45d0fc..27e6171e72 100644
--- a/spock-core/src/main/java/org/spockframework/runtime/extension/builtin/RepeatUntilFailureExtension.java
+++ b/spock-core/src/main/java/org/spockframework/runtime/extension/builtin/RepeatUntilFailureExtension.java
@@ -1,5 +1,6 @@
 package org.spockframework.runtime.extension.builtin;
 
+import org.spockframework.runtime.DataIteratorFactory;
 import org.spockframework.runtime.IDataIterator;
 import org.spockframework.runtime.extension.*;
 import org.spockframework.runtime.model.*;
@@ -8,6 +9,8 @@
 import java.util.*;
 import java.util.concurrent.ExecutionException;
 
+import static org.spockframework.runtime.DataIteratorFactory.UNKNOWN_ITERATIONS;
+
 public class RepeatUntilFailureExtension implements IAnnotationDrivenExtension<RepeatUntilFailure> {
   @Override
   public void visitFeatureAnnotation(RepeatUntilFailure annotation, FeatureInfo feature) {
@@ -32,8 +35,8 @@ public RepeatUntilFailureDataDriver(int maxAttempts) {
     @Override
     public void runIterations(IDataIterator dataIterator, IIterationRunner iterationRunner, List<ParameterInfo> parameters) {
       int estimatedNumIterations = dataIterator.getEstimatedNumIterations();
-      int maxIterations = estimatedNumIterations * maxAttempts;
-      List<Object[]> arguments = new ArrayList<>(estimatedNumIterations);
+      int maxIterations = estimatedNumIterations == UNKNOWN_ITERATIONS ? UNKNOWN_ITERATIONS : (estimatedNumIterations * maxAttempts);
+      List<Object[]> arguments = estimatedNumIterations == UNKNOWN_ITERATIONS ? new ArrayList<>() : new ArrayList<>(estimatedNumIterations);
       dataIterator.forEachRemaining(arguments::add);
       for (int attempt = 0; attempt < maxAttempts; attempt++) {
         for (Object[] args : arguments) {
diff --git a/spock-specs/src/test/groovy/org/spockframework/smoke/extension/RepeatUntilFailureExtensionSpec.groovy b/spock-specs/src/test/groovy/org/spockframework/smoke/extension/RepeatUntilFailureExtensionSpec.groovy
index 9b4fa7c045..ff1508aaa1 100644
--- a/spock-specs/src/test/groovy/org/spockframework/smoke/extension/RepeatUntilFailureExtensionSpec.groovy
+++ b/spock-specs/src/test/groovy/org/spockframework/smoke/extension/RepeatUntilFailureExtensionSpec.groovy
@@ -74,6 +74,30 @@ class RepeatUntilFailureExtensionSpec extends EmbeddedSpecification {
     result.testsSkippedCount == 2
   }
 
+  def "repeats with unknown iteration count"() {
+    given:
+    runner.throwFailure = false
+
+    when:
+    def result = runner.runSpecBody """
+      @Shared
+      int count = 0
+
+      @RepeatUntilFailure
+      def "test"() {
+        expect:
+        ++count < 3
+
+        where:
+        x << [1].iterator()
+      }
+    """
+
+    then:
+    result.testsStartedCount == 1 + 3
+    result.testsSucceededCount == 1 + 2
+    result.testsFailedCount == 1
+  }
 
   def "ignores aborted tests"() {
     when: