diff --git a/src/modbus-rtu-usb.c b/src/modbus-rtu-usb.c
index 4ef08e19..5b781a6f 100644
--- a/src/modbus-rtu-usb.c
+++ b/src/modbus-rtu-usb.c
@@ -418,7 +418,9 @@ static int _modbus_rtu_usb_connect(modbus_t *ctx)
     }
 
     if (ctx->debug) {
-        printf("Number of USB devices: %ld\n", devs_len);
+        // Here and below: cast from ssize_t to a portable type big enough for expected
+        // values
+        printf("Number of USB devices: %lld\n", (long long int) devs_len);
     }
 
     for (i = 0; i < devs_len; i++) {
@@ -433,16 +435,16 @@ static int _modbus_rtu_usb_connect(modbus_t *ctx)
         if (r != LIBUSB_SUCCESS) {
             if (ctx->debug) {
                 fprintf(stderr,
-                        "libusb_get_device_descriptor for device #%ld failed: %s\n",
-                        i,
+                        "libusb_get_device_descriptor for device #%lld failed: %s\n",
+                        (long long int) i,
                         libusb_strerror(r));
             }
             continue;
         }
 
         if (ctx->debug) {
-            printf("Considering device #%ld (%04x:%04x)\n",
-                   i,
+            printf("Considering device #%lld (%04x:%04x)\n",
+                   (long long int) i,
                    dev_desc.idVendor,
                    dev_desc.idProduct);
         }
@@ -461,7 +463,9 @@ static int _modbus_rtu_usb_connect(modbus_t *ctx)
         r = libusb_get_port_numbers(d, ud.port_path, sizeof(ud.port_path));
         if (r < 1) {
             if (ctx->debug) {
-                fprintf(stderr, "libusb_get_port_numbers for device #%ld failed\n", i);
+                fprintf(stderr,
+                        "libusb_get_port_numbers for device #%lld failed\n",
+                        (long long int) i);
             }
             continue;
         }
@@ -477,8 +481,8 @@ static int _modbus_rtu_usb_connect(modbus_t *ctx)
         if ((r = libusb_open(d, &dev_handle)) != LIBUSB_SUCCESS) {
             if (ctx->debug) {
                 fprintf(stderr,
-                        "libusb_open for device #%ld failed: %s\n",
-                        i,
+                        "libusb_open for device #%lld failed: %s\n",
+                        (long long int) i,
                         libusb_strerror(r));
             }
             continue;
@@ -540,7 +544,7 @@ static int _modbus_rtu_usb_connect(modbus_t *ctx)
 
         if (is_match) {
             if (ctx->debug) {
-                printf("Found Device %ld (Path %s):\n", i, path_buffer);
+                printf("Found Device %lld (Path %s):\n", (long long int) i, path_buffer);
                 printf("  Vendor ID: 0x%04x\n", ud.vid);
                 printf("  Product ID: 0x%04x\n", ud.pid);
             }
@@ -548,10 +552,11 @@ static int _modbus_rtu_usb_connect(modbus_t *ctx)
             ctx_rtu_usb->device_handle = dev_handle;
 #if defined HAVE_LIBUSB_POLLFD && HAVE_LIBUSB_POLLFD
             if (usb_ctx) {
-                const struct libusb_pollfd** pollfds = libusb_get_pollfds(usb_ctx);
+                const struct libusb_pollfd **pollfds = libusb_get_pollfds(usb_ctx);
                 if (pollfds) {
-                    unsigned	j;
-                    for (j=0; pollfds[j] != NULL; j++) {}
+                    unsigned j;
+                    for (j = 0; pollfds[j] != NULL; j++) {
+                    }
                     if (ctx->debug) {
                         printf("Got a list of %u libusb file descriptors to poll\n", j);
                     }
@@ -572,9 +577,10 @@ static int _modbus_rtu_usb_connect(modbus_t *ctx)
             }
 #else
             if (ctx->debug) {
-                printf("Can not get a list of libusb file descriptors to poll from this libusb version\n");
+                printf("Can not get a list of libusb file descriptors to poll from this "
+                       "libusb version\n");
             }
-#endif	/* HAVE_LIBUSB_POLLFD */
+#endif /* HAVE_LIBUSB_POLLFD */
             break;
         }
 
diff --git a/src/modbus.c b/src/modbus.c
index b735359e..af3e9781 100644
--- a/src/modbus.c
+++ b/src/modbus.c
@@ -388,7 +388,7 @@ int _modbus_receive_msg(modbus_t *ctx, uint8_t *msg, msg_type_t msg_type)
     if (ctx->s < 0) {
         if (ctx->debug) {
             /* we may not have an FD with e.g. libusb usage */
-            fprintf(stderr, "Using a backend without a file descriptor, will not select() on it.\n");
+            fprintf(stderr, "Using a backend without a file descriptor, will not natively select() on it.\n");
         }
     } else {
         FD_SET(ctx->s, &rset);
@@ -444,12 +444,7 @@ int _modbus_receive_msg(modbus_t *ctx, uint8_t *msg, msg_type_t msg_type)
                 errno = saved_errno;
 #endif
             }
-            if (ctx->s >= 0) {
-                return -1;
-            }
-            // else: We have at most tried some default FD's but not
-            // the (lacking) one for the backend, so fall through for
-            // its recv method anyway (e.g. query libusb directly).
+            return -1;
         }
 
         rc = ctx->backend->recv(ctx, msg + msg_length, length_to_read);
diff --git a/tests/Makefile.am b/tests/Makefile.am
index 0daee73d..61e88332 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -16,10 +16,12 @@ noinst_PROGRAMS += \
 endif
 
 common_ldflags = \
-	$(top_builddir)/src/libmodbus.la \
-	-lusb-1.0
+	$(top_builddir)/src/libmodbus.la
 
 if WITH_LIBUSB
+common_ldflags += \
+	-lusb-1.0
+
 apc_test_client_SOURCES = apc-test-client.c
 apc_test_client_LDADD = $(common_ldflags)
 endif