diff --git a/examples/14_toml_template.cpp b/examples/14_toml_template.cpp
index f595e9b10f..076bf7893e 100644
--- a/examples/14_toml_template.cpp
+++ b/examples/14_toml_template.cpp
@@ -2,6 +2,7 @@
 
 std::string backendEnding()
 {
+    return "json";
     auto extensions = openPMD::getFileExtensions();
     if (auto it = std::find(extensions.begin(), extensions.end(), "toml");
         it != extensions.end())
@@ -50,7 +51,7 @@ void write()
      * Don't specify datatype and extent for this one to indicate that this
      * information is not yet known.
      */
-    E["z"].resetDataset({openPMD::Datatype::UNDEFINED});
+    E["z"].resetDataset({});
 
     ds.extent = {10};
 
@@ -100,7 +101,7 @@ void read()
     openPMD::Series read(
         "../samples/tomlTemplate." + backendEnding(),
         openPMD::Access::READ_LINEAR);
-    read.readIterations(); // @todo change to read.parseBase()
+    read.parseBase();
     openPMD::helper::listSeries(read);
 }
 
diff --git a/include/openPMD/Dataset.hpp b/include/openPMD/Dataset.hpp
index a610ce6a67..ad1125d9d3 100644
--- a/include/openPMD/Dataset.hpp
+++ b/include/openPMD/Dataset.hpp
@@ -41,18 +41,24 @@ class Dataset
 public:
     enum : std::uint64_t
     {
-        JOINED_DIMENSION = std::numeric_limits<std::uint64_t>::max()
+        JOINED_DIMENSION = std::numeric_limits<std::uint64_t>::max(),
+        UNDEFINED_EXTENT = std::numeric_limits<std::uint64_t>::max() - 1
     };
 
-    Dataset(Datatype, Extent = {1}, std::string options = "{}");
+    Dataset(Datatype, Extent, std::string options = "{}");
 
     /**
      * @brief Constructor that sets the datatype to undefined.
      *
-     * Helpful for resizing datasets, since datatypes need not be given twice.
+     * Helpful for:
+     *
+     * 1. Resizing datasets, since datatypes need not be given twice.
+     * 2. Initializing datasets as undefined, as used by template mode in the
+     *    JSON/TOML backend. In this case, the default (undefined) specification
+     *    for the Extent may be used.
      *
      */
-    Dataset(Extent);
+    Dataset(Extent = {UNDEFINED_EXTENT});
 
     Dataset &extend(Extent newExtent);
 
diff --git a/src/IO/JSON/JSONIOHandlerImpl.cpp b/src/IO/JSON/JSONIOHandlerImpl.cpp
index c10d64934f..0dd2239fd5 100644
--- a/src/IO/JSON/JSONIOHandlerImpl.cpp
+++ b/src/IO/JSON/JSONIOHandlerImpl.cpp
@@ -644,7 +644,7 @@ void JSONIOHandlerImpl::createDataset(
         }
         case IOMode::Template:
             if (parameter.extent != Extent{0} &&
-                parameter.dtype != Datatype::UNDEFINED)
+                parameter.extent[0] != Dataset::UNDEFINED_EXTENT)
             {
                 dset["extent"] = parameter.extent;
             }
@@ -1998,7 +1998,7 @@ JSONIOHandlerImpl::obtainJsonContents(File const &file)
             }
         }
 
-        if (m_IOModeSpecificationVia == SpecificationVia::DefaultValue &&
+        if (m_attributeModeSpecificationVia == SpecificationVia::DefaultValue &&
             openpmd_internal.contains(JSONDefaults::AttributeMode))
         {
             auto modeOption = openPMD::json::asLowerCaseStringDynamic(