diff --git a/lib/Service/NegativeSampleGenerator.php b/lib/Service/NegativeSampleGenerator.php
index 6081bd13..6d1ee89d 100644
--- a/lib/Service/NegativeSampleGenerator.php
+++ b/lib/Service/NegativeSampleGenerator.php
@@ -59,7 +59,7 @@ private function getUniqueIPsPerUser(Dataset $positives): array {
 	private function generateFromRealData(array $uidVec, array $uniqueIps): array {
 		return array_merge(
 			$uidVec,
-			empty($uniqueIps) ? [] : $uniqueIps[random_int(0, count($uniqueIps) - 1)]
+			$uniqueIps[random_int(0, count($uniqueIps) - 1)]
 		);
 	}
 
@@ -96,6 +96,10 @@ public function generateShuffledFromPositiveSamples(DataSet $positives, int $num
 		$max = count($positives);
 		$uniqueIps = $this->getUniqueIPsPerUser($positives);
 
+		if ($uniqueIps === []) {
+			return new Labeled();
+		}
+
 		return new Labeled(
 			array_map(function (int $id) use ($uniqueIps, $positives, $max) {
 				$sample = $positives->sample($id % $max);
diff --git a/tests/Unit/Service/NegativeSampleGeneratorTest.php b/tests/Unit/Service/NegativeSampleGeneratorTest.php
index 2b1cd611..f65f8f00 100644
--- a/tests/Unit/Service/NegativeSampleGeneratorTest.php
+++ b/tests/Unit/Service/NegativeSampleGeneratorTest.php
@@ -112,20 +112,9 @@ public function testGenerateMultipleShuffledFromLimitedUnique(): void {
 			self::assertTrue(
 				$ipVec == self::decToBitArray(3, 32) ||
 				$ipVec === self::decToBitArray(4, 32),
-				'sample has a unique IP'
+				'Sample must have an unique IP'
 			);
 		}
-
-		$positives = new Unlabeled([
-			array_merge(self::decToBitArray(1, 16), self::decToBitArray(1, 32)),
-			array_merge(self::decToBitArray(2, 16), self::decToBitArray(1, 32)),
-			array_merge(self::decToBitArray(3, 16), self::decToBitArray(1, 32)),
-			array_merge(self::decToBitArray(4, 16), self::decToBitArray(1, 32)),
-		]);
-
-		$result = $this->generator->generateShuffledFromPositiveSamples($positives, 5);
-
-		self::assertCount(5, $result);
 	}
 
 	/**
@@ -154,11 +143,30 @@ public function testGenerateMultipleShuffledFromUniquesOnly(): void {
 			self::assertTrue(
 				$ipVec === self::decToBitArray(1, 32) ||
 				$ipVec === self::decToBitArray(2, 32),
-				'Sample has an unique IP'
+				'Sample must have an unique IP'
 			);
 		}
 	}
 
+	/**
+	 * Generating shuffled samples isn't possible when no user has an unique IP.
+	 * In that case, we have to return an empty Labeled() object as merging will
+	 * fail otherwise. See GitHub issue #860 for more.
+	 * @return void
+	 */
+	public function testGenerateShuffledFromDuplicatesOnly(): void {
+		$positives = new Unlabeled([
+			array_merge(self::decToBitArray(1, 16), self::decToBitArray(1, 32)),
+			array_merge(self::decToBitArray(2, 16), self::decToBitArray(1, 32)),
+			array_merge(self::decToBitArray(3, 16), self::decToBitArray(1, 32)),
+			array_merge(self::decToBitArray(4, 16), self::decToBitArray(1, 32)),
+		]);
+
+		$result = $this->generator->generateShuffledFromPositiveSamples($positives, 4);
+
+		self::assertCount(0, $result, 'Returned sample must be empty');
+	}
+
 	/**
 	 * @return int[]
 	 */