diff --git a/reader/MarCCD.cpp b/reader/MarCCD.cpp
index 288d244c1..a5ee68d3a 100644
--- a/reader/MarCCD.cpp
+++ b/reader/MarCCD.cpp
@@ -251,13 +251,18 @@ std::vector<std::string> CollectSweep(const std::string &path) {
     return out;
 }
 
+// Asked of every input before anything is read, including inputs that are not marCCD at all, so it
+// answers a question and never fails: a file it cannot open or make sense of is simply not one of
+// ours. The directory arm has to be inside the try as much as the single-file one - it opens the
+// first plausible file in the folder, which can be unreadable, and the throw came out of a call site
+// that does not catch it, ending the run with no message at all.
 bool CanRead(const std::string &path) {
     std::error_code ec;
-    if (std::filesystem::is_directory(path, ec))
-        return !CollectSweep(path).empty();
-    if (!PlausibleExtension(std::filesystem::path(path)))
-        return false;
     try {
+        if (std::filesystem::is_directory(path, ec))
+            return !CollectSweep(path).empty();
+        if (!PlausibleExtension(std::filesystem::path(path)))
+            return false;
         return !InstrumentHeader(path).empty();
     } catch (const JFJochException &) {
         return false;
diff --git a/reader/SMV.cpp b/reader/SMV.cpp
index 405582db1..d818cac6d 100644
--- a/reader/SMV.cpp
+++ b/reader/SMV.cpp
@@ -222,13 +222,18 @@ std::vector<std::string> CollectSweep(const std::string &path) {
     return out;
 }
 
+// Asked of every input before anything is read, including inputs that are not SMV at all, so it
+// answers a question and never fails: a file it cannot open or make sense of is simply not one of
+// ours. The directory arm has to be inside the try as much as the single-file one - it opens the
+// first plausible file in the folder, which can be unreadable, and the throw came out of a call site
+// that does not catch it, ending the run with no message at all.
 bool CanRead(const std::string &path) {
     std::error_code ec;
-    if (std::filesystem::is_directory(path, ec))
-        return !CollectSweep(path).empty();
-    if (!PlausibleExtension(std::filesystem::path(path)))
-        return false;
     try {
+        if (std::filesystem::is_directory(path, ec))
+            return !CollectSweep(path).empty();
+        if (!PlausibleExtension(std::filesystem::path(path)))
+            return false;
         const auto kv = HeaderBlock(path);
         return kv.has_value() && kv->count("SIZE1") && kv->count("SIZE2");
     } catch (const JFJochException &) {
diff --git a/rugnux/rugnux_cli.cpp b/rugnux/rugnux_cli.cpp
index 9c10383ec..69cbb1b70 100644
--- a/rugnux/rugnux_cli.cpp
+++ b/rugnux/rugnux_cli.cpp
@@ -1420,10 +1420,14 @@ static int RunRugnux(int argc, char **argv) {
     JFJochMarCCDReader marccd_reader;
     JFJochSMVReader smv_reader;
     JFJochReader *reader_ptr = nullptr;
-    const bool input_is_cbf = JFJochCBFReader::CanRead(input_file);
-    const bool input_is_marccd = !input_is_cbf && JFJochMarCCDReader::CanRead(input_file);
-    const bool input_is_smv = !input_is_cbf && !input_is_marccd && JFJochSMVReader::CanRead(input_file);
+    // Which format this is. Asked INSIDE the try: these open and read the file - a directory whose
+    // first plausible member cannot be read throws here - and an input the program cannot read has to
+    // come out as the message below, not as an unhandled exception with nothing printed at all.
+    bool input_is_cbf = false, input_is_marccd = false, input_is_smv = false;
     try {
+        input_is_cbf = JFJochCBFReader::CanRead(input_file);
+        input_is_marccd = !input_is_cbf && JFJochMarCCDReader::CanRead(input_file);
+        input_is_smv = !input_is_cbf && !input_is_marccd && JFJochSMVReader::CanRead(input_file);
         if (input_is_cbf) {
             cbf_reader.ReadFiles(input_file);
             reader_ptr = &cbf_reader;