diff --git a/monix-reactive/shared/src/main/scala/monix/reactive/internal/builders/Zip2Observable.scala b/monix-reactive/shared/src/main/scala/monix/reactive/internal/builders/Zip2Observable.scala
index 65535de55..ba639f204 100644
--- a/monix-reactive/shared/src/main/scala/monix/reactive/internal/builders/Zip2Observable.scala
+++ b/monix-reactive/shared/src/main/scala/monix/reactive/internal/builders/Zip2Observable.scala
@@ -154,7 +154,7 @@ private[reactive] final class Zip2Observable[A1, A2, +R](obsA1: Observable[A1],
       def onError(ex: Throwable): Unit =
         signalOnError(ex)
       def onComplete(): Unit =
-        signalOnComplete(hasElemA1)
+        lock.synchronized(signalOnComplete(hasElemA1))
     })
 
     composite += obsA2.unsafeSubscribeFn(new Subscriber[A2] {
@@ -176,7 +176,7 @@ private[reactive] final class Zip2Observable[A1, A2, +R](obsA1: Observable[A1],
       def onError(ex: Throwable): Unit =
         signalOnError(ex)
       def onComplete(): Unit =
-        signalOnComplete(hasElemA2)
+        lock.synchronized(signalOnComplete(hasElemA2))
     })
 
     composite
diff --git a/monix-reactive/shared/src/main/scala/monix/reactive/internal/builders/Zip3Observable.scala b/monix-reactive/shared/src/main/scala/monix/reactive/internal/builders/Zip3Observable.scala
index 0efac6886..40f6fcaa4 100644
--- a/monix-reactive/shared/src/main/scala/monix/reactive/internal/builders/Zip3Observable.scala
+++ b/monix-reactive/shared/src/main/scala/monix/reactive/internal/builders/Zip3Observable.scala
@@ -163,7 +163,7 @@ private[reactive] final class Zip3Observable[A1, A2, A3, +R](
         signalOnError(ex)
 
       def onComplete(): Unit =
-        signalOnComplete(hasElemA1)
+        lock.synchronized(signalOnComplete(hasElemA1))
     })
 
     composite += obsA2.unsafeSubscribeFn(new Subscriber[A2] {
@@ -186,7 +186,7 @@ private[reactive] final class Zip3Observable[A1, A2, A3, +R](
         signalOnError(ex)
 
       def onComplete(): Unit =
-        signalOnComplete(hasElemA2)
+        lock.synchronized(signalOnComplete(hasElemA2))
     })
 
     composite += obsA3.unsafeSubscribeFn(new Subscriber[A3] {
@@ -209,7 +209,7 @@ private[reactive] final class Zip3Observable[A1, A2, A3, +R](
         signalOnError(ex)
 
       def onComplete(): Unit =
-        signalOnComplete(hasElemA3)
+        lock.synchronized(signalOnComplete(hasElemA3))
     })
 
     composite
diff --git a/monix-reactive/shared/src/main/scala/monix/reactive/internal/builders/Zip4Observable.scala b/monix-reactive/shared/src/main/scala/monix/reactive/internal/builders/Zip4Observable.scala
index 379729377..e27144518 100644
--- a/monix-reactive/shared/src/main/scala/monix/reactive/internal/builders/Zip4Observable.scala
+++ b/monix-reactive/shared/src/main/scala/monix/reactive/internal/builders/Zip4Observable.scala
@@ -169,7 +169,7 @@ private[reactive] final class Zip4Observable[A1, A2, A3, A4, +R](
         signalOnError(ex)
 
       def onComplete(): Unit =
-        signalOnComplete(hasElemA1)
+        lock.synchronized(signalOnComplete(hasElemA1))
     })
 
     composite += obsA2.unsafeSubscribeFn(new Subscriber[A2] {
@@ -192,7 +192,7 @@ private[reactive] final class Zip4Observable[A1, A2, A3, A4, +R](
         signalOnError(ex)
 
       def onComplete(): Unit =
-        signalOnComplete(hasElemA2)
+        lock.synchronized(signalOnComplete(hasElemA2))
     })
 
     composite += obsA3.unsafeSubscribeFn(new Subscriber[A3] {
@@ -215,7 +215,7 @@ private[reactive] final class Zip4Observable[A1, A2, A3, A4, +R](
         signalOnError(ex)
 
       def onComplete(): Unit =
-        signalOnComplete(hasElemA3)
+        lock.synchronized(signalOnComplete(hasElemA3))
     })
 
     composite += obsA4.unsafeSubscribeFn(new Subscriber[A4] {
@@ -238,7 +238,7 @@ private[reactive] final class Zip4Observable[A1, A2, A3, A4, +R](
         signalOnError(ex)
 
       def onComplete(): Unit =
-        signalOnComplete(hasElemA4)
+        lock.synchronized(signalOnComplete(hasElemA4))
     })
 
     composite
diff --git a/monix-reactive/shared/src/main/scala/monix/reactive/internal/builders/Zip5Observable.scala b/monix-reactive/shared/src/main/scala/monix/reactive/internal/builders/Zip5Observable.scala
index 49856a371..8beb1a69a 100644
--- a/monix-reactive/shared/src/main/scala/monix/reactive/internal/builders/Zip5Observable.scala
+++ b/monix-reactive/shared/src/main/scala/monix/reactive/internal/builders/Zip5Observable.scala
@@ -175,7 +175,7 @@ private[reactive] final class Zip5Observable[A1, A2, A3, A4, A5, +R](
         signalOnError(ex)
 
       def onComplete(): Unit =
-        signalOnComplete(hasElemA1)
+        lock.synchronized(signalOnComplete(hasElemA1))
     })
 
     composite += obsA2.unsafeSubscribeFn(new Subscriber[A2] {
@@ -198,7 +198,7 @@ private[reactive] final class Zip5Observable[A1, A2, A3, A4, A5, +R](
         signalOnError(ex)
 
       def onComplete(): Unit =
-        signalOnComplete(hasElemA2)
+        lock.synchronized(signalOnComplete(hasElemA2))
     })
 
     composite += obsA3.unsafeSubscribeFn(new Subscriber[A3] {
@@ -221,7 +221,7 @@ private[reactive] final class Zip5Observable[A1, A2, A3, A4, A5, +R](
         signalOnError(ex)
 
       def onComplete(): Unit =
-        signalOnComplete(hasElemA3)
+        lock.synchronized(signalOnComplete(hasElemA3))
     })
 
     composite += obsA4.unsafeSubscribeFn(new Subscriber[A4] {
@@ -244,7 +244,7 @@ private[reactive] final class Zip5Observable[A1, A2, A3, A4, A5, +R](
         signalOnError(ex)
 
       def onComplete(): Unit =
-        signalOnComplete(hasElemA4)
+        lock.synchronized(signalOnComplete(hasElemA4))
     })
 
     composite += obsA5.unsafeSubscribeFn(new Subscriber[A5] {
@@ -267,7 +267,7 @@ private[reactive] final class Zip5Observable[A1, A2, A3, A4, A5, +R](
         signalOnError(ex)
 
       def onComplete(): Unit =
-        signalOnComplete(hasElemA5)
+        lock.synchronized(signalOnComplete(hasElemA5))
     })
 
     composite
diff --git a/monix-reactive/shared/src/main/scala/monix/reactive/internal/builders/Zip6Observable.scala b/monix-reactive/shared/src/main/scala/monix/reactive/internal/builders/Zip6Observable.scala
index 7285bcbb8..9098c82f4 100644
--- a/monix-reactive/shared/src/main/scala/monix/reactive/internal/builders/Zip6Observable.scala
+++ b/monix-reactive/shared/src/main/scala/monix/reactive/internal/builders/Zip6Observable.scala
@@ -181,7 +181,7 @@ private[reactive] final class Zip6Observable[A1, A2, A3, A4, A5, A6, +R](
         signalOnError(ex)
 
       def onComplete(): Unit =
-        signalOnComplete(hasElemA1)
+        lock.synchronized(signalOnComplete(hasElemA1))
     })
 
     composite += obsA2.unsafeSubscribeFn(new Subscriber[A2] {
@@ -204,7 +204,7 @@ private[reactive] final class Zip6Observable[A1, A2, A3, A4, A5, A6, +R](
         signalOnError(ex)
 
       def onComplete(): Unit =
-        signalOnComplete(hasElemA2)
+        lock.synchronized(signalOnComplete(hasElemA2))
     })
 
     composite += obsA3.unsafeSubscribeFn(new Subscriber[A3] {
@@ -227,7 +227,7 @@ private[reactive] final class Zip6Observable[A1, A2, A3, A4, A5, A6, +R](
         signalOnError(ex)
 
       def onComplete(): Unit =
-        signalOnComplete(hasElemA3)
+        lock.synchronized(signalOnComplete(hasElemA3))
     })
 
     composite += obsA4.unsafeSubscribeFn(new Subscriber[A4] {
@@ -250,7 +250,7 @@ private[reactive] final class Zip6Observable[A1, A2, A3, A4, A5, A6, +R](
         signalOnError(ex)
 
       def onComplete(): Unit =
-        signalOnComplete(hasElemA4)
+        lock.synchronized(signalOnComplete(hasElemA4))
     })
 
     composite += obsA5.unsafeSubscribeFn(new Subscriber[A5] {
@@ -273,7 +273,7 @@ private[reactive] final class Zip6Observable[A1, A2, A3, A4, A5, A6, +R](
         signalOnError(ex)
 
       def onComplete(): Unit =
-        signalOnComplete(hasElemA5)
+        lock.synchronized(signalOnComplete(hasElemA5))
     })
 
     composite += obsA6.unsafeSubscribeFn(new Subscriber[A6] {
@@ -296,7 +296,7 @@ private[reactive] final class Zip6Observable[A1, A2, A3, A4, A5, A6, +R](
         signalOnError(ex)
 
       def onComplete(): Unit =
-        signalOnComplete(hasElemA6)
+        lock.synchronized(signalOnComplete(hasElemA6))
     })
 
     composite