diff --git a/src/libslic3r/PrintBase.cpp b/src/libslic3r/PrintBase.cpp
index 812e060c6d..81affe04d4 100644
--- a/src/libslic3r/PrintBase.cpp
+++ b/src/libslic3r/PrintBase.cpp
@@ -64,7 +64,7 @@ std::string PrintBase::output_filename(const std::string &format, const std::str
 		boost::filesystem::path filename = format.empty() ?
 			cfg.opt_string("input_filename_base") + default_ext :
 			this->placeholder_parser().process(format, 0, &cfg);
-        if (filename.extension().string() != default_ext)
+        if (filename.extension().empty())
             filename = boost::filesystem::change_extension(filename, default_ext);
         return filename.string();
     } catch (std::runtime_error &err) {
diff --git a/src/libslic3r/SLAPrint.cpp b/src/libslic3r/SLAPrint.cpp
index 06c4f687b9..ebe7b74027 100644
--- a/src/libslic3r/SLAPrint.cpp
+++ b/src/libslic3r/SLAPrint.cpp
@@ -597,7 +597,9 @@ void SLAPrint::finalize()
 std::string SLAPrint::output_filename(const std::string &filename_base) const
 {
     DynamicConfig config = this->finished() ? this->print_statistics().config() : this->print_statistics().placeholders();
-    return this->PrintBase::output_filename(m_print_config.output_filename_format.value, ".sl1", filename_base, &config);
+    // we need to remove the extension from the format string or the following call to PrintBase::output_filename() won't be able to change it to ".sl1"
+    std::string format = boost::filesystem::path(m_print_config.output_filename_format.value).replace_extension("").string();
+    return this->PrintBase::output_filename(format, ".sl1", filename_base, &config);
 }
 
 std::string SLAPrint::validate() const