diff --git a/backend/app/src/test/kotlin/io/tolgee/api/v2/controllers/v2ImportController/SingleStepImportControllerTest.kt b/backend/app/src/test/kotlin/io/tolgee/api/v2/controllers/v2ImportController/SingleStepImportControllerTest.kt
index 9cda1f5746..de4e5a6842 100644
--- a/backend/app/src/test/kotlin/io/tolgee/api/v2/controllers/v2ImportController/SingleStepImportControllerTest.kt
+++ b/backend/app/src/test/kotlin/io/tolgee/api/v2/controllers/v2ImportController/SingleStepImportControllerTest.kt
@@ -109,6 +109,27 @@ class SingleStepImportControllerTest : ProjectAuthControllerTest("/v2/projects/"
     getTestTranslation(namespace = "test").assert.isNotNull
   }
 
+  @Test
+  @ProjectJWTAuthTestMethod
+  fun `maps null namespace from non-null mapping`() {
+    val fileName = "guessed-ns/en.json"
+    performImport(
+      projectId = testData.project.id,
+      listOf(Pair(fileName, simpleJson)),
+      getFileMappings(fileName, namespace = null),
+    ).andIsOk
+
+    getTestTranslation().assert.isNotNull
+
+    performImport(
+      projectId = testData.project.id,
+      listOf(Pair(fileName, simpleJson)),
+      mapOf(),
+    ).andIsOk
+
+    getTestTranslation(namespace = "guessed-ns").assert.isNotNull
+  }
+
   @Test
   @ProjectJWTAuthTestMethod
   fun `respects provided format`() {
diff --git a/backend/data/src/main/kotlin/io/tolgee/service/dataImport/CoreImportFilesProcessor.kt b/backend/data/src/main/kotlin/io/tolgee/service/dataImport/CoreImportFilesProcessor.kt
index 089ab23e9c..0b2de76c6c 100644
--- a/backend/data/src/main/kotlin/io/tolgee/service/dataImport/CoreImportFilesProcessor.kt
+++ b/backend/data/src/main/kotlin/io/tolgee/service/dataImport/CoreImportFilesProcessor.kt
@@ -156,7 +156,7 @@ class CoreImportFilesProcessor(
     val mappedNamespace = findMappedNamespace()
 
     if (mappedNamespace != null) {
-      return mappedNamespace
+      return getSafeNamespace(mappedNamespace)
     }
 
     if (this.namespace != null) {
@@ -174,6 +174,12 @@ class CoreImportFilesProcessor(
   }
 
   private fun FileProcessorContext.findMappedNamespace(): String? {
+    if (mapping != null) {
+      // if mapping is present, use it even when the namespace is null to avoid
+      // guessing from the file name
+      return this.mapping?.namespace ?: ""
+    }
+
     return this.mapping?.namespace
   }